I have and older system with a HD4850 and a NvidiavGTS250. It works for productivity stuff like excel on the AMD card and access on the nvidia card.
I have also been able to run games on the AMD while running steam, coretemp, and an avi on the nvidia. I don't know if 3d vision would work, but I dont see why not.
The games start on whatever card the primary monitor is plugged into. You can really freak out the drivers by changing the primary display in windows, or setting the the 2 displays to "Duplicate This Display".
But for the most part the 2 cards work fine in my old core 2 rig.