Mac Pro, 2x 2.8 Quads, 10GB ram, 512MB video, 2x 24" lcd, plain old two button optical mouse (sorry, Apple, but the MIghty Mouse just doesn't work for games)

I like being able to see every window without any switching, so I run on two screens. Main client full on primary display at 1920x1200, four child clients on second display at 960x600. Now there is a little bit of overlap on the four small windows because of their top bars, but it's small enough to not bother me.

I use CloneKeys for key broadcasting, and agree with TheBigBB's assessment. Slow and steady wins the race. I'll occasionally have one client not receive a keystroke if I get going too fast.

And yes, I run all on Mac OS. I don't see why anyone would possibly want to Boot Camp over for WoW. (I did Boot Camp it to run the upgraded graphics EVE client and still feel like a bitch for doing so...)

Depending how I feel once I get up through the lvl 80 instances, I might graduate up to 10 boxing. If so, I'll pop in a second video card and get two more displays. Again that's mainly because I just want to be able to see everything at once. I think the Pro will handle 10 without even blinking. I often transcode video while playing with no problems. Can't let that CPU power just sit idle!