Do NOT get a AMD graphic card. AMD has a driver limitations so it can only render 1 3d source at one time correctly. So you cant run more than 1 wow with a AMD (ATI) card.
I don't know where you get your information but I have an ATI Radeon with 2gb memory and two GPUs and I can run my main screen at 55 fps and 15-20 on all of my slaves on a second monitor. I only get some lag in Shattrath due to the fact that I'm only running 32bit Vista. The Radeon replaced a 780MB 8800GTX Nvidia card and the difference is huge. Are you sure you work for ASUS and not Nvidia?