The Pentium D has EM64T, so yeah, it will run. The only advantage is for the extra 64 bit registers...the biggest thing you'll notice is the ability to use more RAM. The unfortunate side-effect, though, is software support for it. Driver support for 64 bit is especially pathetic. If I was you I'd go 32 bit unless you were running a server or something.
PC EYE, that is exactly what I wanted to do. Thanks for the advice. So, When I install Vista, simply unplug the XP HDD and install vista on the separate sata HDD? Then once installed I can use the BIOS to decide what disk to boot from? Sounds like the way to go man. I will still see the XP drive after booting though won't I? Well, that won't be too bad I guess. Thanks again!
Better to modify the boot.ini to be able to dual boot.