I two box at the moment on a 24" 1900x1200 and I run PiP. Main is full 1900x1200 (full video settings), and the clone account in a small 500/375 (or 600/450?) window with a 800x600 game rez (all video settings turned down) in the very bottom right corner and I find it works quite well. Once they catch up to my level 70 on another account I plan to purchase a 19-20" widescreen monitor and have the 2 clones on the 2nd monitor in portrait mode for tri-boxing.

Mine is similar to http://solidice.com/keyclone/layouts...egions_pip.jpg but with the clone window moved all the way to the bottom corner (and a different UI layout).