I agree that XP x64 looks like it's the best Windows choice for any new hardware. I just have no trigger time on it at all. With luck I will get it running under Bootcamp pretty soon on a new Macbook...
Here's a fairly decent MS article (.doc file) on x64 that spells out the whats and wherefores in plain english:
http://tinyurl.com/yxpxmo
To me, Vista says "stay away" in different ways. The two messages I get loud and clear are: ridiculous hidden resource overhead for DRM, and super-stupid security "fixes".
x64 is a kind of surprising and timely option. I kind of wish I had more time and boxes to get a PC native evaluation install running. Soon enough I hope, one way or the other.
-r