*cracks fingers*
Ok , to start xp x64 is NOT XP, its server 2003 x64 in PRO mode, thus many of us from "the scene" (not that scene, the windows geek scene) call it x64pro .
Incompat issues are RARE these days, drivers are as good or better then the 32bit xp drivers, and far better then vista drivers.
Let me start by saing x64pro will allow you to access as much ram as your sysem can hold but will run FAR better on lower ram ammounts then any version of vista would, your very unlikely to find modern hardware u cant get x64pro drivers for, i have 1 item out of all the STACKS of hardware i own that dosnt got a driver an OLD webcam that came with my x800xt pe from asus way back b4 x64 os's where avalable.
I have found maby 4 apps that wouldnt work by default, all of them where easy fixes, one was doom3, it was just an MSI file that needed edited because Id fucked up and didnt add any os to the file other then XP, but if you know about doom3 u know u dont need the installer, just copy the files off the dvd to your hdd and its playable