First let’s see how faster the SLI tandem of two GeForce GTX 680s is in comparison with one such card:
At 1920x1080 the GeForce GTX 680 SLI is an average 67% and 75% ahead of the single card with antialiasing off and on, respectively. The difference is larger at 2560x1600: 67% and 83%. And if we take only the most resource-consuming games that correctly support multi-GPU technologies, the SLI efficiency will be as high as 100%!
Now let’s compare the GeForce GTX 680 SLI with the GeForce GTX 690, the latter being a dual-GPU graphics card with two slightly down-clocked but full-featured GK104 Kepler chips.
The GeForce GTX 680 SLI is somewhat faster than the single GeForce GTX 690 in most of the tests, which might be expected considering their specs. The SLI tandem is an average 1.4 to 1.9% faster at 1920x1080 and 2.6 to 2.8% faster at 2560x1600. We should keep it in mind, however, that these numbers refer to average frame rates whereas the GeForce GTX 690 was often better in terms of the bottom frame rate.
The last pair of summary diagrams help us compare two multi-GPU solutions: GeForce GTX 680 SLI (2x2 GB, 1006/6008 MHz) and Radeon HD 7970 CrossFireX (2x3 GB, 1000/5800 MHz).
The Radeon HD 7970s are faster in Aliens vs. Predator (2010), Sid Meier's Civilization V, Tom Clancy's H.A.W.X. 2 and Total War: Shogun 2. In the rest of the games the two solutions are comparable or the GeForce GTX 680s win. Nvidia is an average 7-13% ahead of AMD at 1920x1080 and 3-18% at 2560x1600 across all the tests.