I have no idea what you are talking about. If you need to run two monitors you can use the stock card for 2 24'' lcd's or 1 30'' lcd and 1 24'' lcd. So anything smalled you can run two of anything. Do external video cards even exsist? Highly unlikely.
There is no such thing as an external video card that I know of. The only one that I can think of would be nVidia's Quadplex (the external quad x16 PCI-E Quadro enclosure yum), but that requires a PCI-E slot to plug into the PC and I don't even think it works on Apples anyways.
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