If I install a second graphics card for a quad monitor setup, how powerful does the second card have to be?

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http://serverfault.com – I have a dual monitor machine at work that I use to write code, run scripts, and plot results. The amount of plotting can be quite demanding. I have ran out of video memory a couple of times (its either 1 or 2 gb). I wan't to extend this setup to have quad monitors by adding a another graphics card, but I am unsure of a few things. The more I think about it the more I realize I have no idea how this is supported at the lower level. I know that I won't be using SLI, (these questions only apply to the case where you have two separate models of graphics card plugged into the pci-e and not co (HowTos)