Well they help alot, its just most people have the wrong idea of SLI. When your running SLI, each video card is responsible for 1 half of the screen. They arent used as REAL dual video cards, such as processors. In simply terms, you dont double the graphics. Since each card is only responsible for half the screen, its very easy for your GPU to run demanding games.
Thus, you can play games at extremely high resolution, and you will have more FPS.
Finished With TF.