Crossfire. While SLI does scale better, crossfire is more forgiving, AMD has Eyeinfinity,which I hope to put to good use when I get 2 more Dell Dell U2410 monitors and AMD has DX11 . For example, you can have 3 or 4 of just about any Radeon card that has two crossfire connectors, even a 4670, but on the nVidia side only the GTX 260 and above will allow for more than two GPU's. On the AMD side, motherboards supporting SLI are a little more problematic than boards supporting crossfire. On the Intel side, boards supporting SLI cost extra for the privilege.
But given the choice, I'd go with a single card.
I have just upgraded from a 2 HD 5770 Crossfire to a single HD 6970, got it cheap, but I was more than happy with the Crossfire set up.
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