First of all, I want to make it clear that this is written from a Canadian standpoint, and I'll be using Canadian dollars. Not like it's much different anymore!

I'll also be referencing Canadian shops and Canadian availability. Your kilometre...age.... something... may vary. For buyers in USA, I'd recommand Tigerdirect.com (fast processing and shipping) or or of course, NewEgg (I've never dealt with them since they're fascists who won't share their stuff, but they have the best prices usually).
My favourite stores are TigerDirect.ca and NCIX.com. I'll be referencing them a lot. By the way, this guide is written with budget in mind - if you want to just throw dollars at a problem like computer performance, buy something from Alienware and be done with it.
This guide also assumes you have some degree of technical knowledge. Do not attempt to assemble a computer on your own, not the first time anyway. And if you're going to, don't do it on a carpeted floor. While the computer is plugged in, touch the case to ground yourself and eliminate any static electricity before proceeding. An anti-static wristband might be a good idea.
First, a motherboard.
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The big name in motherboards right now is Asus. There are others, such as MSI, Gigabyte, and so on, but let me put it this way: I've owned 9 Asus motherboards, every single one was perfect. In the last year, I've bought 5 motherboards from other brands - two had to be returned, and a third was "strongly mediocre", but not return-worthy.
For this reason, I will recommend Asus motherboards over others, but I'm probably just unlucky and other brands are often significantly cheaper. Do you really want to risk the most important component being imperfect though?
To decide on a motherboard, the first thing you need to know is what chipset you want. Nvidia chipsets sometimes have better performance and features, but frankly Nvidia isn't a chipset manufacturer first and foremost - Intel is. People who have "NForce" motherboards (nvidia's brand name), generally got suckered into it by a salesman who says "well it's designed by nvidia, your computer will be faster."
Let's dispose of that myth - chipset makes almost no difference compared with other, more important, factors. Factors, such as CPU, RAM, GPU, and even hard drive.
What chipsets are for is ONE thing - features. If someone even says something about "better performance" from a chipset, they're newbies without a clue - the same people who buy "gaming" or "sli-certified" RAM and "Vista" CPUs... The difference between all current midrange motherboard, performance wise, is unnoticable without complex testing.
Basically, if you get an NForce motherboard, you'll almost certainly get SLi capability - which means, if you want to spend $1500 on two GPUs, you'll get a 50% performance increase under the best circumstances. 2x cost for 50% performance increase isn't worth it to me. Besides which, NForce always seems to have been plagued with one problem or another... it's not a mature product, despite having been around for years, and that should tell you something about what Nvidia's real priorities are.
The current "major" chipsets are:
Intel P965 - Good chipset, its only lacking is the PATA controller. This means that you'll need SATA hard drives and CD ROMs, and no floppy drive. But you always want a floppy drive for troubleshooting purposes... So the solution to this is to find a motherboard that has a secondary chipset which features PATA connectors. This is not to be confused with the G965 chipset... it has onboard graphics, which you *don't* want.
NForce 650i - OK chipset, OK price
NForce 680i - OK chipset with too many unnecessary features, bad price
PATA is the old standard for data connections on hard drives, optical drives, floppy drives, etc. You'd recognize it by the wide ribbon-like cable. SATA is a new standard which offers... well they say it offers better speed but I have yet to see real proof that this improves your computer's performance under normal circumstances. The connectors are better though, so it's not a total loss. The main advantage to SATA is that it allows you to configure multiple hard drives into what's called a "RAID" array. RAID arrays are multiple hard drives configured to act as a single one - this means that you can have 4 250 gb hard drives linked together to get a 1 tb hard drive. Another RAID configuration allows you to mirror drives, so that if one of your hard drives fails, you can replace it without any problems. That's the big benefit in my opinion.
RAID jargon (skip this paragraph if you don't plan to use RAID): RAID 0 means the aforementioned multiple-disks-combined-to-one-huge-one configuration. This gives you lots more room for legal music, legal movies, and lots of other legal stuff. But as an added benefit, all the data is split up across all the drives in the array (when possible), which means that under ideal circumstances, having 4 drives means four times the performance. Unfortunately if any one of those drives fails... the whole array is shot. You'll need to start from scratch. That's where RAID 1 comes in. Under this configuration your data is split up, but into identical mirrored copies on multiple disks. 4 disks gives you the same writing speed, but four times the reading speed. As long as one of the mirrors survives, no data will ever be lost. RAID 5 is entirely more complicated. Ehh... it's not for general consumers... the performance decrease is too much, and the benefits are too few. Basically it's a combination of RAID 0 and 1... but... complicated... and messy... good for servers and stuff, I understand, but not regular people. RAID 01 (alternately known as RAID 10) means a combination of RAID 0 and 1. You need at least 4 disks for this configuration, and it's a good compromise of price, performance, and reliability. Basically you split all your data across two drives, then mirror those two drives. As long as you don't lose both drives from one of these "sets", you're fine. 4 disks gives you 2x the write speed and roughly 4x the read speed under ideal circumstances. The downside is that you need more disks.
I use a 4-disk RAID 01 array comprised of 250gb drives, and a fifth standalone 500 gb drive for backup. This gives me 1 terabyte of storage, with lots of performance on my RAID array and somewhere to store all the stuff I'm not currently using on my standalone drive.
You need a RAID controller in order to use a RAID array though. Usually you can get a RAID controller built onto your motherboard.
Besides chipset, the last consideration is CPU socket type... fortunately right now it's a no-brainer. There's colossal number of CPUs available for socket 775 (aka "LGA775"), with a full range of price and performance options.
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Moving right along... You have two choices: buy a motherboard/CPU bundle, or buy them seperate. Depending on circumstances and the deal you get, either way might be a good option. Just BEWARE of limited return allowances. You might get a motherboard/cpu combo that must be returned together - then what happens if the mobo's fine but the CPU's no good? Always check return availability and factor it into your decision.
So what you're looking for is:
An Asus motherboard with an Intel P965 chipset with a secondary chipset which features at least one PATA and floppy controller, and accepts socket 775 CPUs, preferably with an onboard RAID controller. That's narrowed it down!
In fact, at TigerDirect, you're down to 5 choices as of the time of my writing this:
http://www.tigerdirect.ca/applications/ ... =3&Recs=10
Alternatively, if shopping for a CPU+Mobo combo, you have 7 choices:
http://www.tigerdirect.ca/applications/ ... =3&Recs=10
Reasonably-priced options include:
Asus P5B motherboard: (A good base-model board, doesn't have really good RAID support, but otherwise good)
http://www.tigerdirect.ca/applications/ ... CatId=2432
Asus P5N mobo with E6400 Core 2 Duo CPU: (Includes SLI... unfortunately its crappy Nvidia chipset doesn't let you use fast RAM...)
http://www.tigerdirect.ca/applications/ ... CatId=2406
EDIT: A better price on the P5B:
http://www.newegg.com/Product/Product.a ... 6813131030
EDIT: P5B-E:
http://www.newegg.com/Product/Product.a ... 6813131070
Yessir, you have TWO choices at this moment... At TigerDirect anyway. If you want to look for another deal someplace else, here's your list of must-have features besides those already mentioned:
-Accepts Core 2 CPUs
-Accepts 1066 mhz FSB (front-side bus)
-Accepts PC2-6400 RAM (that's 800mhz), or PC2-4200 (that's 533 mhz) as a bare minimum
A good example is the P5B-E, which I have. It's basically the same as the basic P5B, just a better chipset, meaning better RAID. Early P5B-E motherboards, however, do not allow RAM voltage over 2.1 volts, which can be limiting in some cases... I don't know for sure if that applies to the basic P5B. I have a feeling it doesn't.
In order for P5B motherboards to fully recognize Core 2 Duo CPUs, you'll need to flash their bios. This is a very simple process thanks to Asus.
So that's motherboard out of the way. If you find a deal, post it for others and I'll also give you my best guess about whether it's good or not. Frankly, $175 for that P5B is nothing to write home about... I'd look for a better price elsewhere. $175 including tax would be more reasonable.
So that's motherboards out of the way!
Next comes CPU. This is fairly easy... You want an E4400 if you're going budget, or an E6600 if you want better performance. The cost difference s fairly significant, but so is the performance difference.
Ehh.. for a long time, I've been saying, "wait for July 22 before buying a CPU" because a price drop was scheduled for that day. Well.. it would appear the price drop hasn't hit Canada yet, so I'll have to link to those dirty commie ******** at NewEgg to show reasonable prices:
E4400: http://www.newegg.com/Product/Product.a ... 6819115014
E6600: http://www.newegg.com/Product/Product.a ... 6819115003
Another option is Pentium D. This is super-budget, decent performance. These are becoming outdated and are cheap. So cheap, you'll feel guilty thinking about them if you're involved in a serious relationship. I mean, CHEAP. $60 for a low-end one, $120 for a high-end one. Pentium D is basically just two Pentium 4s on the same chip. Good for multitasking, otherwise becoming outdated... still amazing value though. I might almost recommend buying one and seeing if it's adequate for you. CHEAP. Runs HOT though, very hot. HOT and CHEAP. You're despicable.
Curiously, us Canucks seem to have a better price than NewEgg on the P-D 805 CPU:
http://www.tigerdirect.ca/applications/ ... No=2204069
Why not go with AMD, you ask? Go ahead and try it if you want. AMDs are currently super cheap, and almost capable of competing with Core 2s in many cases. Unfortunately they also run kinda hot and I have no experience building AMD systems, so this guide would be for naught. Go for the fastest one you can afford, preferably of the FX (double core) series.
More MHz does not equal more speed. A low mhz Core 2 is faster than a high-mhz Pentium 4. Finally, remember that Pentium 4-based CPUs run frickin' hot. Like 50 degrees at idle sometimes... That's crazy. Get too fast of a Pentium 4 and you'll have to spend more money on cooling equipment than you spent on the CPU.
Next comes RAM. If you're buying a Core 2, here's how it works... take the CPU's rated FSB (front side bus) speed, divide it by two. Now buy RAM that runs at that speed. So for instance, if you're buying an E6600, it has a rated FSB of 1066, so buy 533 RAM. That's PC2-5300. If you're buying an E4400, buy 400 mhz ram, which is PC2-4200. This will make your RAM, motherboard, and CPU all run in sync, which brings some performance benefit and could save a headache later if you plan on overclocking.
Your options in RAM include:
PC2-4200: (533 mhz)
http://www.tigerdirect.ca/applications/ ... CatId=2262 (buy 2 kits, giving you 2 GB of RAM)
http://www.tigerdirect.ca/applications/ ... CatId=2262 (buy 1 kit, giving you 2 GB of RAM)
If you're going to use faster RAM, use MUCH faster RAM or your performance loss due to unsynchronized CPU:RAM will outweigh the benefit. I'd recommend going to PC2-6400, if anything.
http://www.tigerdirect.ca/applications/ ... CatId=2264
(Buy 2 kits, 2 GB)
http://www.tigerdirect.ca/applications/ ... CatId=2264
(Buy 1 kit, 2 GB)
http://www.tigerdirect.ca/applications/ ... CatId=2264
(Buy 1 kit, 2 GB)
Recommended RAM brands are Patriot, Ultra, Corsair, and OCZ, in that order.
Finally! GPU! GPU! GPU!
Your choices here are limitless, so I'll just give you guidelines:
Prices vary WIDELY between shops, shop around or wait for a special.
The two big heavy hitters are the GeForce 8000 series, and the Radeon X2000 series. Geforce wins, though Radeon comes close and can usually be found cheaper.
The very best cost:performance ratio can be found on the GeForce 8800 GTS 320 mb card. You won't need more than 320 mb of video RAM any time in the near future, and the 8800 GTS can be overclocked without risk to a point where the difference between it and a stock-speed 8800 GTX is negligible.
Do NOT go for the 8600 or 8300 cards. They're not worth it. If you haven't got the cash for the real deal, go for something in the 7000 series. The 7800 GT for instance, is an excellent card which will serve you well for some time, as long as you don't downgrade to Windows Vista. Windows 3.1 to Windows Vista is a downgrade. Even Windows ME to Windows Vista is a downgrade. Do not use Vista for any reason. Ever. If you want reasons, go ahead and ask, but take my at my word, you don't want that trash. It is the biggest flop in operating system history, and the only people who deny it are the ones that bought it and can't admit to themselves they wasted $300 that would've been better used as kindling.
So anyway... Here are your two best bets:
7900 GS:
http://www.newegg.com/Product/Product.a ... 6814143070 ($136)
8800 GTS 320mb:
http://www.newegg.com/Product/Product.a ... 6814130082 ($260)
Why not ATI? Because they have crappy drivers... not a really big issue but Nvidia's always a safe bet.
The last major item is the power supply. All I can say is, read user reviews, read web site reviews, read all the reviews you can, and get a power supply that is more than you need. I'd recommend a 600 watt power supply as a minimum. Get one that's know for running cool and quiet, with a very long life. Get a cheap one and it'll die after a month or something stupid, but not before the ball bearing wears out on the fan, treating you to a grinding noise for the last weeks of its life.
Did you read all that? If you did, congratulations, you're probably the only one...
So let's figure cost. At this point, I've given in to the sweet temptation of just using NewEgg as reference, since their catalog is so much easier to search through and their prices are so much better...
Asus P5B Motherboard: $113
Geforce 7900 GS Video Card: $136
2 GB RAM: $66
Intel Core 2 Duo E4400 CPU: $127
Power Supply: $80 minimum
Total is $522 plus shipping.
There are two places you can reduce the cost, and nowhere else IMO: the motherboard and CPU. A Pentium D CPU could cost as much as $80 less, and a non-Asus motherboard could cost as much as $50 less.
Do not settle for less than a 7800 series GPU for any reason. Below that, the performance drops off like a cliff and the price only goes down slowly. And if a certain person tells you that you should get something from the GeForce FX series, since that's all you really need, report him to the FBI - he's a terrorist trying to ruin your computer enjoyment and waste your money.
These are also retail prices. Find a reliable eBay or Amazon seller or just shop around and you can probably reduce the costs by a further 10-20%.