This is NOT an SLI rig. But I can run it in a 'hybrid mode' where I'm essentially getting access to 4 monitors. I try to make sure that I always run my main Revit instance on the monitor that is plugged into the 'good' FX 580 video card.
The main lesson here is that you need to install GeForce drivers to make this all happen properly. The Quadro drivers do not seem to work very well in this hybrid setup.
I am running 275.33 drivers, which you can download from here:
GeForce 275.33 Driver
This GeForce driver also includes cool stuff like overclocking options in the Nvidia Control Panel.