In general which is a better option? Taking into account things like driver support as well. I hear in general Nvidia have much better drivers for multi gpu setups but it might just be fanboys saying that. I'm talking about the 2gb version of the 6950 btw.
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I would expect both to be pretty evenly matched at 1080P or below, but the pair of 6950s would have the edge at higher resolutions/multiple monitor scenarios....