I'm running Q6600 @ 2.4 ghz, 8gb ddr 800mhz ram, HD 4870 x2 2gb video, and play wow with sound disabled.
I play on a Samsung 305T: 640x480 (x4) along the top, with 1446x1085 main window, and a space open for FireFox, Ventrilo etc.

I 5-box on one machine, using Keyclone.
My background clients are limited to 10 FPS, my main window to 40 FPS.

With graphic settings all the way down on every client, except that the initial main has distance maxed but everything else minimal...
WoW runs smoothly everywhere except Shatt, which is definately playable if not perfectly smooth.

My only issue is with taking Flight Paths. Roughly half the time, one or two of my four smaller windows will freeze.
When this occurs, Keyclone will open (un-minimize) but reports as "not responding".
The only way to get it to work is a re-boot, even with ending task in task manager it will not restart.
This issue is new to 3.x did not have it in 2.x