I have a radeon hd 7770 ghz ed and I have noticed that I have a small fps decrease than when I was on one monitor. One of the monitors is 1920x1080 which is what I use to game. The other is at 1440x900 which is used for multitasking in game and such.
The drop isn't noticible until I turn the settings up in game and a steady fps is more critical.
Could the 2nd monitor be what is dropping my fps, since it is having to display more pixels?