If you're buying a new machine, choose Vista Home Ultimate (32 bit).

There's absolutely nothing wrong with it and it's more than capable of running multiple instances of WoW.

Somewhere in the forums, there's a post about my own experiences with Vista when I first adopted it on two of my 6 machines.

Having been told with great authority by somebody on these forums that it was impossible, I ran 5 instances of WoW on a new 2Gb Vista notebook. The memory usage was marginally lower than the same scenario on an XP desktop and there was no perceptable difference in performance.

I have limited experience with the 64 bit Microsoft offerings, but there's anecdotal evidence that they will cause you more pain than gain due to driver availability and support, so I avoid them myself.

Buying a new machine with an old OS makes no sense to me, although as you may have noted from Keyclone's comment, he does not support his application under Vista so that may be a consideration for you.