Running Ultimate Vista 64 and I got no trouble with it,
I just have to disable the aero theme because of some driver issue with my videocard (atikmdag crashes)

I've heard (but can't find confirmation atm) Vista 32 got it's 4gig support in SP1,
it can allocate 4gig, but if you have1 gig on your videocard it has to allocate that aswell, so in the end only the ~3 gig ram works.

[EDIT] tss, tssk, videocards with 4gig :s I've been working to long today