I'd avoid XP 64 if I were you, I tried it and had nothing but problems with it, Vista 64 works much better and, importantly, is much more stable (not often I say that about Vista compared to XP), couldn't say about Windows 7, which won't run on my comp, but if that works I'd stick with it personally.

I'd just like to say as well, I'm so envious of that machine of yours I could explode. :cursing: