Added a second GeForce 260 GTX and after reboot, enabled SLI, only to go immediately to a black screen with a blinking cursor at top left, requiring a hard reboot - came back up SLI not enabled. Nothing I tried worked. Searched the net to find all manner of suggestions including swapping vid cards, checking bridge, swapping memory, etc. and etc. and etc. and finally found a guy who has solved this. So, if you add a second card, or buy two cards for SLI and have this problem, here's the solution. Open Nvidia Control Panel and set your desktop resolution to 1024x768. Then, enable SLI and the screen will go black, but come right back. Then, reset desktop resolution.
I was running an older version of Nforce drivers and getting hard locks in COD with this new setup at highest rez., but upgrading to current drivers seems to have fixed that. And so, this graphic edge, added to my already considerable and renowned skill, leaves me a serious force to be reckoned with. Now, me kill you long time.