NVIDIA just released a new WHQL (Microsoft certified) driver for their GeForce series graphics cards. Any GeForce card to date starting from series 6 and upwards seems to be supported.
Note: Quad SLI technology with GeForce 9800 GX2, 3-way SLI technology, and Hybrid SLI technology are only supported on Windows Vista.
We have a discussion thread open on these drivers right here
Have you read the Guru3D SLI (3-way and 2-way) review ?