yeah , what he said ^^^^
I personally run a 20 incher and a 15 incher side by side, and another 17 and 15 incher side by side, and they work great. Ones new, the other is ancient in both cases.
I run matrox dualhead cards with support independent refresh rates and resolutions on each monitor...i'm not sure if the radeon has the drivers for this or not...if not, then you have to make both monitors into one "big desktop" in which case it can be annoying to have one really big and one really small monitor.