I would definately go with the Vista 64.
XP 64bit was never fully debuged, and it is based of of a version of windows server.
And for the person who said "i think everybody agrees that vista isnt worth it"
i don't know what your talking about.
better automated troubleshooting, better GFX for gaming and SP1 for Vista is even out now.
but i do agree partially with Flakes, Vista64 & Ubuntu
Cause that is what i run mostly