I recently ordered two GeoForce GTX 560Ti video cards as part of a build that I am doing, but I can't get the SLI to work. Whenever I try, the NVIDIA control panel stops responding and the graphics cards stop working. From that point, the computer's graphics run directly from the motherboard and I can't get the graphics cards back on unless I completely power down and switch off the system. I'm running on windows 7 with a Samsung SyncMaster BX2440 monitor.
start with gpu-z and check that you have the video cards in the two top slots. those are the two 8x slots. with gpu-z check that the cards have the same hardware id..bios..speed and ram..if there not the same sli wont turn on. also see that the mb reading the card right.