How fast/expensive is your machine, that you can play multiple instances of WoW, with at least one on full graphics at 1920x1200? And is that 50FPS in combat with lots of mobs, or just standing still away from people?
I have a similar computer to you, with one exception. I've got a 8800gtx. I believe that 768mb of ram on the gfx card helps a ton with multiboxing on one pc since you are basically rendering a game at 3840x1200 res. (the additional speed of the gtx helps too i'm sure) Quadcore and 4gb of ram rocks, but dualcore and 2gb of ram seems to get the job done with a good enough gfx card.

I get 45-60 fps in the AH in stormwind and the other 5 are pegged at 20fps