start with gpu-z and read both cards. they should be the same speed and the same or close device id for sli to run.with gpu-z make sure both pci bus are reading right. also check that your mb suports sli a few low end ones dont. some of the newer cheap z77 have one pci slot at 16x and the other slot at 4x. for sli to run right two cards should be running at 8x speed or 16x speed.