Just a quick note. I have Vista 64 with 6 gigs of ram, 8800gts, and a 6600 quad core. It's about 20% slower than my Vista 32 bit system with 4 gigs (3.5 usable) of memory. So don't expect Vista 64 bit and more memory to solve anything. It has to translate everything from 32 bit to 64 bit which causes overhead.

What was helpful for me was sharing 1 wow folder and a 10K RPM raptor hard drive. When you go into shat it has to load all the player models for each character in the city (and on each instance of wow) - that's why when there is no one in shat it probably works fine, but when lots of people are on it slows down big time.