I believe that your device driver is not defending the 6600GT card. For this i would like to advise you to change the device driver file and will require to admit the device ID of a 6600GT card. This will help you in setting up the 6600GT card. I have also attempted the result on my organization and it is functioning fine. So you can also try to do the same thing. I hope that it will definitely help you in solve your trouble and you will be successfully able to associate the 3rd monitor on your organization.