win7 64 for me... it really only makes a difference if you have more than 4 gigs of ram. the other issue is that make em different is that win7 64bit uses large addressing so every float number is 8 bytes instead of 4 on 32bit so the o.s. is about 30 percent bigger in the system. if the game has a 64bit exe you will often get a smoother experience just down to more effective processing of large numbers. also you can run 32bit game on 64bit O.S. but not the other way round so more compatibility... then again if retro gaming is your thing you will have an easier time with 32bit as it can handle older 16bit installers where 64bit wont.
Agree 64bit hands down. Worth pointing out that although it is often said that you need 64bit to make use of MORE than 4gb, you can't even use 4gb with 32bit. If you install 4gb you will only get about 3gb, which is a bit tight these days.