This is my setup, I use 30" screen fro main, and 24" for slaves. My main toon is running in 2560x1200, so not only i have much wider view, but mainly IM doing it like this so i can have some desktop space on the top, for itunes, keyclone etc.
My slaves are in 960x600, i just like the bit of extra space fro mods and whatnot. In shat Im getting about 30 fps all across the board, its smooth and there is no slow downs, every where else im at 50-60.
(btw Im kinda poor in this ss lol, I just bought 4 epic flying mounts)

http://img359.imageshack.us/img359/2393/uipn8.jpg