I have a AMD X2 (4000+) OC'd to 2.5Ghz, 4GB, and a HD3870XT and I can run 5 clients - Main at default video settings, slaves use /maxfps at 15.

I'll admit that when I pull the entire graveyard in SM, my FPS drop to about 5 though I dont care though, as I'm just dropping totems and and healing myself so any lack of FPS is a non-issue.

Interestingly my video card never shows more than 25% load though.