I am adding a third monitor to my current setup, and therefore I will need to buy another graphics card. Do I have to get the same kind of video card as the one I have now? I currently have a GeForce 8800 GT. Does it have to be the exact same, or will any other GeForce work? Or could it be any GeForce 8000 series?
No, it does not need to be the same model, or even the same brand.
I have a few clients using competing brands of video cards in one computer to support multiple displays, and it seems to work fine for them. This doesn't mean that two specific brands/models might not be compatible though (you'll have to test for driver conflicts; make sure that whoever you're buying from knows this ahead of time).
I have found it's more likely to work and to work with less hassle if you get two cards that will both operate off the same driver. The Nvidia drivers tend to behave better than the drivers that come with the card.
Also, the driver disks often don't understand multiple-card configurations and will not work or will produce messed-up results. Downloading just the driver itself is more reliable.
With related cards only once have I not succeeded--I had two virtually identical cards, no go--and when I called tech support one of their technical people (not merely a support person) insisted that Windows didn't support multiple cards.
With unrelated cards I'm below 50% on getting it to work and in one case involving motherboard video + one card we found we had to flash the system BIOS down to an earlier version!