I fixed the issue. I had to install coolbits and modify the value to 18.
I just wanted to add my input that I was having the exact same issue, two different sized (800-ish MB & 1700-ish MB) GTX 275 EVGA cards and I couldn't get the SLI option to appear in the Nvidea control panel. Thanks to the Coolbits advice I got my damned GTX275's to SLI. =) I tried just editing the reg file, and restarting, that didn't work, but after downloading the coolbits 2.0 reg hack from guru 3D and after restarting, got the damned cards to work! =)