I use XP 64 in work. The sound works perfectly on the onboard sound card of my HP XW8400 and the drivers are all on the HP web site.
I have tried Vista 64 but went back top XP after around a month or so. More speed and much more reliable. Will move to windows 7 64 soon though.
I have a problem with my onboard device that can't be resolved, so I'm looking for a new one. I'm hoping to find someone with a working card that isn't onboard because at this point I'll take anything that will get me sound.