While i like a lot of the things in vista, using hardware more effectivly isnt one of the things it do very well.
At least not if a majority of the benchmarks are to be believed. Now they did fix a lot of issues in Service Pack 1, but a well optimised xp still run most aplications better then a well optimised vista. Now when it comes to x64 its very hard to compare vista with xp since vista have both and XP only have 32 bit, the x64 XP is actually based on windows server 2003.
When using 3d aplications i would say that x64 is a MUST, not being able to allocate more then 2gb ram to any single process is very limiting.
So for optimal speed and performance i would go with XP x64, for usuability and overall "good feel" i would go with Vista x64.
Not going to bring up linux since it cant really run any decent aplications for CG in the first place.
But if things work well and there is no need for more speed then id go with vista any day over XP, since it much less of a hassle to configure and secure properly, and it looks way better :)