I installed 64bit vista a few weeks ago when I upgraded my ram to 4gb. As far as drivers go, pretty much everything worked and the system is stable. Only problem I ran into was with the iphone. There are no iphone drivers (or itunes) for vista 64... figures.

I have no experience with xp 64, but I've heard most hardware companies didn't bother writing drivers for it (and they all need to be signed by ms or something?) If your going with 4gig of ram, your best option is 64bit vista I'm afraid.