SLI is currently not supported for 6 and 7 series cards.

As detailed in the release notes:

Adds support for GeForce 8800 GTS 320 MB
-Beta driver for NVIDIA SLI support for GeForce 8800 GTX/GTS GPUs
-This driver supports the following features:
-Single GPU support
-DirectX 9 support for GeForce 6/7/8 series GPUs
-DirectX 10 support for GeForce 8800 GPUs
-OpenGL support for GeForce 6/7/8 series GPUs
-NVIDIA SLI support
-DirectX 9 support for GeForce 8800 GPUs
-OpenGL support for GeForce 8800 GPUs
-DirectX 9 and OpenGL NVIDIA SLI support for GeForce 6 and 7 series GPUs and DirectX 10 NVIDIA SLI support for GeForce 8800 GPUs will be available in a future driver

http://www.techzonez.com/comments.php?shownews=20423