I've tried Win 7 both 32bit & 64bit. Win7 32bit is more compatible with existing Microsoft applications (Office 2003/2007) and seems to be quite stable. If you're into downloading stuff like movies & TV shows, BitTorrent clients work better in Win 7 32bit than in Win7 64Bit OSs.
Win7 64 Bit still requires a whole heap of drivers for application software that's been around for some time. Some software venders are not bothering to upgrade drivers for 64bit OSs.
In principle, I would say stick with XP for a while if possible, but if you have to get a later version, then give Vista a big miss and go for Win 7 32bit.

Opinion is based on my experience.

Cheers & Good luck!