I have a 280 card, I'm currently running two monitors set at 1680x1050.
I have used the main screen at 1920x1200 but I kind of think things run more smoothly with both windows are of the same resolution.
right now I only have 3 WoWs running on this machine and two on my other machine.

I tried 5 wows on one machine but I really didn't like it, I never have lag with two machines.