I just recently (two days ago) installed 64-bit Vista Ultimate on a partition in my rig. So far its all good (once I nixed the UAC!!!). I spent seceral hours last night getting Firefox set back up like I have it on XP (sans a few extensions that 3.1b2 covers... the backing up a profile trick didn't work).
I've still got a ways to go yet, but I am getting there. Once I have Vista set up just right I will cut the strings to XP and fully switch, keeping XP on a spare drive in case of utter catastrophe.