I'm running Vista Ultimate 64bit version and I can't be happier. System is:

AMD x2 3800+
BFG Nforce4 Ultra Mobo
2gb pc2700 Ram ( I need to upgrade it to 3200 that the x2 supports)
BFG 7600GT O/C
160gb and a 80gb Hd.

I was hesitant to upgrade to it, and I wouldn't have if I had to buy it. But my dad had a spare copy he gave me, so I figured hey couldn't hurt. I personaly like the pop ups, lets me know what is going on in the comp. The only thing I don't like is apple is not currently supporting cd burning through itunes with this, but thats not really Vista's fault. Runs all my apps perfectly, and it is way "prettier" then xp. Memory needs to be there in abundance though but I'm just gonna upgrade to the 4gb when I feel like it. As it sits it does everything I want/need it to.