You're suppose to be able to get both if you're getting the retail version. If you're buying an OEM version...hmmm, I'd say 64-bit. Why? Well, supposedly Crysis, which happens to be the benchmark game right now, outputs about a 10-15% performance increase in the 64-bit version vs. the 32-bit version. Also, if you're running only 2 gb of ram, 64-bit really becomes pointless.

So, I'd say stick with 32-bit XP for stability for now, and wait out until DX10.1 gets implemented. However, do realize that a lot of games coming out are being optimized for Vista, in the case of Crysis, Vista 64-bit. Supposedly, the .1 version of DX10, will implement something that will give a huge increase in speed to games utilizing DX10 graphics, that's just the word on the street, haven't seen anything conclusive. Also, the 8800GT will not be able to utilize the 10.1 feature.

 

I'm running a quad core Intel on an ATI HD3870XT and it runs UT2k7 at 1680X1050 with Hardware Physcs and Vsync enabled on Vista 32Bit and it runs flawless....

Same rig ran like shit on XP...soo....not sure...my Dual Core systems run fine on XP though...but your on AMD so...I got no friggin clue....