On Vista, X2 6000, 2GB Ram...

I can run 2 copies of wow with zero performance hit. Using the /timetest I got identical results. Both @ 1280x1024 on separate heads of the video card.

Adding a third gives a significant punch to performance. Since I know Vista takes more ram to run base OS than XP I'd say that anything over 3 will likely cause problems on XP and 5 would be really pushing it with only 2gb of ram.

I can get 5 up and running but if I try and do anything significant I have major issues.