Your rig beats the snot out of mine so I'm really not sure what to say. I run 4 on the following
Intel Core 2 Duo 2.66ghz
4GB ram (3.5 in xp, yay)
PNY 7800GTX video (single)

I'm even running in dual view on 2 monitors in portrait mode, which several have said drop the framerate, and I run at 1024x768 stretched with WoWMaximizer.

It may be just in your video settings. On mine I pull the terrain distance, spell detail, EVERYTHING back to the minimum because 80% of the time I'm looking at my main machine which is all bright and perty. The same should go for you, just keep the settings however you like on the window that's your 'main' and cut the settings back on the others.

That may be obvious and already done but I just can't think what else could be bogging you.