Interesting article. I'm stuck with "only" two monitors at the moment but one thing that I have found is: If you are designing an application for end users (either web based or desktop) then one of the monitors should be no bigger than the average size used by the target audience. UIs that look great on a 1600x1200 monitor often just don't cut it on a 1280x800 screen, and many users still have smaller screens.