For studio work I'm using a Dell Precision 690 (Woodcrest quad core) which has Vista x64 loaded. I'm actually really impressed with how smooth Vista x64 is, although I've never tried Vista x32, but its really alot nicer, smoother, and far mroe stable the XP ever was. I could never go back.
Live and all my plugins are running really well. No crashes or issues. I'm shopping for a sound card and there are a few that support x64 Vista drivers. MOTU, RME, and EMU are the ones I'm aware of. Anyone have any suggestions?