Stick with XP... Although the Vista Aero interface looks good, tumbling windows, transparency et al looks good, and the security is theoretically better (but a pain, always asking for permission - so you end up ignoring it)
XP is solid, reliable and works for me. (I'm having a painful job rewriting a solid app (about 500000 lines of c# to make it work with Vista)
Plus there are a few security issues with Vista including local escalation of privileges - should be a patch out for it real soon)
Most businesses are avoiding Vista like the plague - they buy licences for it as part of the MS Upgrade pogrom but I haven't heard of ANY sucessful transitions.