Lot of us heavy gamers are on Vista 64 - no issues I have encountered so far and that seems to be the consensus. Games are written in 32 bit and 32 bit software simply runs in the 32 bit mode of your 64 bit capable processor so generally this process is seamless and has no performance hit. Going WAY back, ollder 16 bit games will not run unless maybe you are using a virtual environment.
The big benefit of running 64 bit Vista at this time is the ability to actually use 4 gig of ram or more. I run 8 gig myself because ram is so darn cheap, why not? 64 bit Vista is HOT right now and will probably either become the standard or else become an alternate standard to 32 which will receive 100% support - it's actually quite close to 100% now.
There are a very few exceptions here so YMMV but I wouldn't even think about setting up a Vista box at this time without using 64 bit.