I run 5 boxes on one machine with - compared to others in this thread - very modest hardware:

Core 2 duo E8200 @ 2.66
4GB ram (3.25 is usable)
GTX260
1920x1200 resolution

I can run my main window with maximum settings (and also a few tweaks to go past max settings - render distance is up, more ground cover, etc) and my 4 slaves on low without problems except when it comes to loading everyone to Dalaran (so I use Shat for my home base )

What I do is I throttle the FPS depending on whether the window is the active one - if the window is active, I cap it at 60 FPS, if it is inactive/in back, I cap it at 10 FPS using console commands. If I didn't do that, I'm sure I'd have a real issue with performance. Works like a charm!