I would recomend you get a good size (@22" or so) and make sure that the resolution is high enough to view the game windows you want without dithering.

For example, I recently bought 2 19" 1440x900 monitors, and was unhappy with them because I was viewing four windows on the 2nd. The smallest Wow will render to is 800x600... if the window is any smaller than that, it will be dithered. It was impossible to read combat logs, or even quest text in the quad windows without moving them to the primary window.

So... if your want to display 4 windows, make sure you have a 1600x1200 or better.

I ended up ordering two 22" 1920x1200 to replace the 19" pair. I would have preferred two 1600x1200's, but I couldn't find a model I liked for a price I could afford.

Good luck!