I have recently upgraded from XP to Vista Ultimate 64Bit.
I chose Vista 64Bit because I now have 4 Gig of ram installed and Vista 64 takes full advantage (unlike the 32 bit), especially while using ZB.
Yes I have some compatibility issues, but those issues will be solved soon with driver updates etc.
My system specs are:
EVGA 680i Mainboard
4 Gig Corsair Dominator 6400C4 Ram @ 900mhz
Dell 24 Inch Monitor
150 Gig Raptor X Hard Drive
E6300 C2D CPU overclocked to 3.15ghz
8800 768 GTX Graphics Card
The only problems I have are with my X-FI soundcard and a few games, which is to be expected as the games were written for XP.
My advice is that if you plan on using over 2 Gig of Memory then go for Vista 64, but check out the websites of your GFX card, important software etc first to make sure they have 64 bit drivers, or at least plan to release some.
Things will only get better with Vista 64, and I can honestly say that Vista64 + 4 Gig and ZB 3 are a lovely combination! My system is still highly responsive working with 18 million polygons (not HD, just normal polys).