1st Please do this, this is very common.
Disable SLI in the control panel. Reboot.
Go to device manager, and remove the hardware.
Reboot (make sure that when you come back up, the screen resolution is 800x600 this will make it evident that the driver was infact removed). If iy's not, the driver was not removed properly and the system is still using it. This is less a problem with Vista, but XP sometimes hangs on to the driver.
Install your new driver.
Go to your NvidiaControl panel, and enable sli. (Reboot even if it doesn't aks to)
After you reboot, make sure to go into the Nvidia Control Panel, and make sure that SLI is on.
If it is, run the bench and report back. As stated before, make sure that your power options are set to max performance. Don't run any other programs while running the bench. Do not OC the card yet.