I had an issue once with CSS running SLI. I removed one of the cards and it was fine. If CSS is the only game you run then SLI tbh is overkill.
However. saying that, i installed the latest NVidia drivers
from there site, the Win 7 ones and it worked fine in SLI after that.
What drivers are you using?
Its definatly a graphics card/driver
issue. Have you used SLI with vista/xp and all worked ok in SLI?
If so, im betting its the drivers