There are so many versions of Blender out there, which one should I use for Spring-based games?

http://www.blender.org/download/get-blender/ -- this one?

Your operating system and processor architecture should allow you to pick the right one I hope.

Get the latest and greatest... I think people only keep using the old versions because they got really used to the old interface and with Blender that is a huge deal... making art in Blender is like playing StarCraft... sure anyone can do it but then you see those pros do it and you're like "WTF... how did you... what?... holy hotkeys batman!"

So, if I am contracted to do buildings for Spring 1944 and a hitherto still unpublished game, all I have to do is to get the newest version of Blender and get a version of GIMP with dds generation capabilities? brilliant. Thanks guys, /c hardcore on Kingdom of Loathing has nothing on you

Also another question:

If I create sprites for Zero-K or Zero-K variants like Imperial Winter, what is the preferred poly count? currently I am working on Rise of Nations, using 3ds Max, and for obvious reasons my poly count cannot be more than 900 ...

Imperial Winter is not a Zero-K variant.

Sprites are images which always face the camera, they don't have "polies"

The spring engine doesn't care 'bout polygons. Polygons are not the limiting factor:
viewtopic.php?f=9&t=20762&hilit=polygon

Yeah... unit count, pieces, and textures all tend to bottleneck the CPU before the poly-count does... that said... probably want to keep it under 32,767 to be safe.