Video card performance with 2 screens?

If I play a video game with a dual head 9800gtx nvidia card, does it hurt performace to drive 2 screens cloned? i'm running a projector and a monitor, and want the best performance, but want to see both screens....|||2 screens cloned is not really taxing the graphics card since resolution is not very high. It is extended display (panoramic mode) that really taxes performance due to the resulting very high resolution of the two displays combined.|||.