Compared to the performance of the HD 7970 the speed boost isn't huge, but on the whole the GTX 680 has the edge.
Looking at the Crysis 2 bench though the AMD card has the lead, but that is still within benchmark variance.
In overclocked trim, via the EVGA Precision tool, performance does step up a decent chunk, but the SLI performance is a little less impressive.
That's likely down to early drivers but we didn't see the longed for 2x speed boost we've sometimes seen from SLI combinations.
DirectX 11 tessellation performance
DirectX 11 gaming performance