Quote from ChartingMarkets:
I just did what you said but with 4 monitors. I had a Dual ATI Radeon 9700 Pro Card in the AGP slot using 2 monitors. Then I went out and bought the Dual ATI Radeon 7500 and put it into an open PCI slot. Due to the fact that the ATI cards all run off of the same driver the computer recognized everything just fine and now 4 monitors are connected to one computer.
We have had good results with Matrox (on windows OS) and the newer ATI cards.
We almost always run one brand of card on a box: We have some setups as described here: Boxes with more than one version of newer generation ATI cards in mixed AGP and PCI slots. These work without issue on our standard dell workstation boxes under Windows 2000, XP, and on Linux