If you're getting video errors and you've got STOP 0x124 errors (hardware) - I'd have to suspect issues with your video card.
Can you run each card individually (I'm not real well versed in SLI)? If so, try each one - and run this free video stress test on each one: FurMark: Graphics Card Stability and Stress Test, OpenGL Benchmark and GPU Temperature | oZone3D.Net
I'd post over in the video card forum for more expert advice on the SLI: Graphic Cards - Windows 7 Forums
From what little I do understand about SLI, it's either one card or the other, or it's the connection between the two. Dunno if SLI will work with 2 different cards either.
Just to be sure, I'd remove the current drivers
and install the latest WHQL certified drivers from nVidia.
FWIW - the STOP 0x124 error is a hardware error that's received by your CPU and is passed to Windows. Difficult to analyze - and the results of the analysis don't contain much useful information.
I've had one when I didn't install a video card correctly in a Dell computer (large card, cramped case). Reseating it fixed the STOP error for me.