Well, I use Vista 64bit as my OS, and I've had no problems. All my games work, drivers, software. etc etc etc. I know that I can't speak for everyone in these regards though. The only thing you really need to worry about is the Anti-virus software, but that'll only be round the corner. Besides, Avast, NOD32 and others already support it.
I'd still stick with Vista 64bit though. It's fully compatable with 32bit software and Vista is the future. XP is now old and starting to rust. All new software will be developed with Vista in mind first and foremost.