But would two 560s beat a 5970?
Or even two 6970s which the 6990 could be?
Depending the game dual GTX460's overclocked to around 800-850 can best a HD5970 but it all comes down to driver support (the 460's scale better thus making up the single GPU performance deficit).
A GTX560 based dual GPU card *should* be faster than a HD5970 (thanks again to better scaling) but it would depend heavily on drivers, the game itself and whether the GPU's can run at full speed or not.
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