You'll be looking at a 10-20% (max) increase in performance running Sli, you're far better off saving the coin and getting an epic single card that utilizes dual GPU's on a single card.......
For reference, look into the Nvidia GeForce GTX 280, Nvidia GeForce 9800 GX2 and the ATI Radeon HD 4870 X2.
For better cost/performancethough, look into the ATI Radeon HD 4870. Best bang for buck in a high end environment.
See how you go anyway........ :2thumbs: