I'm sort of at an impasse here as well. I never had a reason to install the xp x64 version I had lying around, but I now have 4GB of ram. The only knocks I saw against it were that there were some driver issues sometimes with hardware, and certain older games had 16 bit installers sometimes. The few lists of games tested I saw were pretty complete.
At any rate, I decided to install it on my second hard drive and I'm going to gradually try to phase it over to my main install and see how it goes. Video card drivers seem to come out regularly alongside their 32bit counterparts.