You will definately want 2 gigs of quality ram(with low timings if you go AMD) if you're serious about gaming. 7900 GT is a very good bang for the buck so don't waste your money on something cheaper that gives only a fraction of its performance.
Get a PSU from FSP/Fortron-Source or Silverstone as these are both powerful and quiet. Speaking of which, go with a Seagate or Samsung harddrive unless you want a computer that sounds like your washing machine.
A lot of the integrated audio solutions you find on motherboards these days really suck arse with a big A.
If you enjoy good music/movies and have a decent set of headphones/speakers you'll want to look into a seperate soundcard. The audiophile would go with a Creative X-fi. The entusiast would pick a Audigy3/Audigy SE which is a good bang for the buck.
Also, make sure you get a decent cooling setup. Buy an expensive air cooler for the cpu and a few 120mm fans to put into the chassis. This way you wont have one of those brand new systems that for some reason just keeps restarting every 1› hours into Prey *cough* overheat *cough*.
