SLI Anti-Aliasing Performance
We didn't perform a comprehensive test routine to assess the performance of all of NVIDIA's SLI AA modes with the GeForce 7900 series, but we did run a couple of tests to get a general idea as to how the various modes perform. NVIDIA informed us that some data is now transmitted across the SLI bridge when SLIAA is enabled on the 7900s, freeing up some bandwidth across the PCI Express interface. Upon first hearing this news, we assumed SLIAA performance could be significantly enhanced, but that did not turn out to be the case.
SLIAA performance improved by between 4% and 9% with the GeForce 7900 GTXs, versus a pair of 512MB GeForce 7800 GTX cards. The biggest gain came in Half Life 2 when enabling SLI 16XAA, where a pair for GeForce 7900 GTX cards were able to outperform a pair of 512MB 7800 GTXs by almost 9 frames per second. ATI's "Super AA" implementation with CrossFire clearly offers higher performance though because all of the frame blending is handled by the compositing engine, which allows each Radeon GPU to run at full speed. With NVIDIA's architecture, the GPU itself handled the offset and blends necessary for SLI AA to function.