If you are aiming to run 64 bit processes you need Vista.

I'd hadn't realised (and haven't checked) XP doesn't have a 64 bit version??? I've been using 64 bit OS (linux x86_64) on PCs since 2005 and didn't think that was anything new then.

How's it go with Win on the x86_64? Do you then have to have all 64 bit progs or is it like Linux with 32 bit backwards compatibility?