Windows XP 64, is one of the most stable OS youll find, I've been using xp64 in work as well as linux 64bit, and to be honest in my personal experience windows XP64 is more stable. Over the year ive run the system apart from a few mandatory restarts (windows updates
) the system never went down or had any issues (the machine was used in film production and gets absolutely hammered by Maya and other 3D software).
Games are hit and miss on any 64bit OS, with Stalker theres a hack to run the exe so its more stable, Colin Mcrae rally 2005 just wont run (copy protection drivers are incompatible) but other than that.. everything runs fine. I would go so far as to say it feels a little quicker than its 32bit little brother.
The obvious advantages concerning memory management are a massive plus for heavy duty professional applications like Maya, Photoshop and After Effects. all using the extra memory perfectly fine.
If your looking for a very very solid OS windows XP 64bit is a great choice - drivers are now mature and there are plenty of them (still worth checking all your hardware has the drivers before commiting though)
As far as Vista 64bit.... well its vista isnt it.. so its unstable, buggy .. but looks nice
personaly i would still consider vista a beta product, and although driver support is considerably better than windows XP 64bit was when it was released - theres still a lot of random applications and drivers which either dont exist or are buggy.
Personal advice would be.
Windows XP 64bit = Best Windows OS currently available and by far the most stable.
Windows Vista 64bit (any version) = wait for service pack 1 before commiting.
On a final note.. .just because you have a 64bit CPU .. doesnt mean youll be getting any more juice out of it .. unless your using applications which require or benifit from the additional ram - you only gain Stability not speed.
Cheers
ROfu