If you are setting up a brand new machine I would recommend Vista-64 just so you can push the RAM up to 8GB. The 32-bit RAM limit is being pushed already when 5-boxing WoW on a single machine and it might use even more with the expansion. I also saw some comments on the CPU that it doesn't like Vista-64 so even if you decide to go Vista-64 at a later date that CPU might screw you.

EDIT: You might also want to consider getting 2x 2GB RAM modules instead of 4x 1GB modules. If you decide to upgrade to 8GB you have 2 slots free on the MB and can just plug in another 2x 2GB.