Just some of my personal experience for extra input.

I currently run 1 full screen WoW on my laptop, Alienware m9700. I actually got two running yesterday but I tend to get an error when I try to run 2. Yesterday somehow the 2nd one ran but when I tried a third I got the error. Anyway, none of that matters, that's just why I only run one instance on this machine.

On a second machine I run 4 instances of WoW windowed and quartered on a single LCD. I haven't tried a 2nd monitor for that PC yet, and probably won't, those toons are relatively easy to control so all I need is a small open window to manually control them if the main dies or I lose one of them or something.

Performance on each window is really surprising. They are each getting 50 fps.

That machine has a dual core AMD at 2.6 GHz, 2GB of ram, and an 8800 gts.