personally, unless there are some big compatibility issues, i'd go x64. After driver support catches up, having x64 will be more desirable, so I'd rather just put up with a few little problems now than have to buy 32bit and then buy x64 later, especially since you can't upgrade to x64 without wiping. As for gaming and performance, it's pretty good. I haven't noticed any sluggishness at all with the 71.84 WHQL drivers, and the NGO Optimized 76.71s beat my previous best 3DMark05 score by like 150 points (it scored 4150, versus 4002 with 76.45s on XP 32bit). I'm using the 76.71s right now. The only game i've heard of that doesn't work is Chaos Theory, and it still installs fine, so I'm sure there will be a patch to fix it. Oh, and Farcry gets added eye candy in x64.
Anyways, the few problems I have are 1) The only AV tool available is Avast, which is okay since it's free. Norton is on the way, as is AVG. 2) Rivatuner doesn't work, so unfortunately you can't force OpenGL 1.5 to get riddick to work. According to the forums, somebody is working on it as we speak. That's really about it. So I'd say go for x64 if your starting from scratch, but it's not a big deal either way.