There is an issue with all 3870 and 4870 ATI Cards, driver independent: If you run with 2 monitors and you rotate your main display by 90 degrees in any direction, your system will start stuttering once you move any 'graphics' window into another display. I confirmed this behavior with EQ2 & Civilization IV. We didn't have a copy of WoW around, so we didn't try it on our regression box... With a pair of 9800 GTX nVidia cards, we did not see this failure. ATI is aware of the issue, no eta for resolution.

Otherwise, I run a pair of 4870's in MultiMon mode and run 6 instance EQ2 on it, works like a champ... So Wow should eat it up. g'luck