I'm currently running a Geforce 260 powering two monitors (36" LCD TV at 1080P) and a 22" monitor at 720P (max resolution for the monitor). Am I hampering my performance due to the card having to input two different displays?
I normally keep my gaming screen on the 36" and my webpages open on the 22" (play wow and look up items on the other monitor).
My other question, is that I play most of my games in a windowed mode. Is this also lowering my performance? Would I be better off in full screen?
It depends on teh game whether or not fullscreen or windowed mode will have a difference, windowed I think typicaly has better performance. You could downloaf Fraps to measure your FPS. Personally I have a dual 1680x1050 setup and I have measured a small ~2% performance hit while running extended mode while in a game vs one screen, not that much of a problem.