for MMOs you probably wouldn't need to. the performance would already be well above 60fps, but if you plan on playing other games it does add a fair bit of performance.
you want an odd number when going a multi monitor setup, which is why 3 is the current norm. if you use 4, you'd have bezels right through the center of the image.
you should be able to drive 3 monitors in EQ2 with a single 5870. again though, if you want to play anything else then a second video card would help retain a good framerate at maximum graphical settings.
what im saying about the even numbers, is that the center of the image will be broken up by the edges of the monitors. this will make playing very annoying. by using 3 monitors you have a central unbroken image and then peripheral vision.
if you want bigger, rather than a wider, image i would suggest going for 3 monitors in portrait mode. head over to widescreen gaming forum if you need any help on that.
If you are running two seperate windows to run your two accounts at once on the same computer, you will not gain any performance at all going crossfire. Crossfire does not work in windowed mode. If you alt tab to the other account, then it may give you a boost.
Before you consider the change, determine your current FPS. Back in the first year of EQ2, it was pretty demanding on systems, but it might not today, unless they updated it. If you find you are already getting 60+ FPS, it would not be worth going crossfire regardless if it's in windowed or fullscreen.