What about game performance? For example, I read that Bioshock ran better in XP regardless of if Vista was running DX9 or DX10 in the Bioshock thread. I haven't read very many XPv64 vs Vista 64 comparisons though, so I couldn't be too sure.
I'm also thinking about waiting until Vista SP1, since as far as I know there are still small but significant bugs in Vista when compared to XP.
-Vista forgets all individual folder view settings?
-reduced network (LAN only?) speed when playing any audio
-Slower file transfer rates
Also, the Vista situation with the Audigy 2 ZS seems rather bleak without EAX; I've even read integrated sounds better in Vista. In fact, this may be the only case where the Vista (x64) drivers are infinitely worse than the XPx64 drivers for my hardware, although it's more an issue of Microsoft overhauling handling of sound than simply driver updates..
Basically, in the short-term of say, one year, is it really worth going to Vista (Ultimate) x64 over XP Pro x64? I've yet to be really compelled Vista x64 is a better road than XP x64.. most of the responses here have been about drivers, but in actuality at present the XP drivers seem much better than the Vista drivers, and at least one of them (Audigy 2 ZS) isn't likely to change in the foreseeable future.