I recently switched GPUs from an HD3650 to a 9800GTX+, and because I also have an old 8500gt, I now have two nVidia cards. This means that in Windows Vista, I can run two video cards (Vista requires the two cards to use the same drivers, but W7 does not). To get the setup working, I had to do some modifications to the 8500gt, as well as build a stand for my monitors, since my desk is too small for three.
The first problem was that both my video cards use PCIe X16, and my computer only has 1 x16 slot. Fortunately, it has four x1 slots, and all speeds of the PCIe interface are electrically compatible with each other. But an x16 card will physically not fit in an x1 slot. To get around this, I just cut off the extra pins on the video card.
The 8500gt's cooler was also very loud, so I put the passive cooler from my old 3650 on it. The holes in the PCB for mounting the cooler were too small for the push pins the heatsink used, so I had to drill them out to get the cooler to fit.
Here's a great example of how useful this setup is: I can have a Photoshop window in full screen on my center monitor, have all the tools open on the right monitor, and have a browser window open on the third. This setup gives me a total resolution of 4224 x 1080.