My current specs for my PC, enable me to run 5 instances of WoW, at 100% CPU usage.

The FPS is smooth, and its quite playable. I have not tested a major city yet however.

EVGA 8800 GTX, 4GB RAM, 2.66GHz Core 2 Duo.

Check this post out. You might find some information.
http://www.dual-boxing.com/forums2/viewtopic.php?t=1595

I use 2 monitors, as it is a huge help when 5 boxing. Its on Horizontal Span.