Image quality

Because of the aforementioned limitations of Quad SLI, NVIDIA recommends Quad SLI setups for users who want to run SLI anti-aliasing with high levels of anisotropic filtering. In these cases Quad SLI can deliver a significant improvement in performance over a more traditional GeForce 7900 GTX SLI setup. Of course, you’re not going to see double the performance of 7900 GTX SLI due in part to the slower clocks, but it is a noticeable improvement.

NVIDIA supports three SLI anti-aliasing modes for Quad SLI: the traditional 8x and 16x modes that are provided by all SLI-capable cards, and a special 32xAA mode that’s unique to Quad SLI. With a Quad SLI graphics setup providing up to 96 pixel shaders and over 150GB/sec of peak memory bandwidth on tap, the premise is that you can not only have the sharpest image quality available on the PC thanks to these SLI-AA modes, but you’ve got enough pixel-pushing horsepower to run that eye candy with the highest frame rates available on the PC as well. Let’s take a look at some screenshots so you can see how much better SLI-AA looks over the more traditional 4xAA setting:

0xAA/16xAF

4xAA/16xAF

8xSLIAA/16xAF

When looking at the above screenshot, the most obvious area to see the difference between 4xAA and 8xSLI-AA is the large tree in the scene above: (note that this is a static tree, i.e. the branches don’t sway in the wind, which could affect direct comparisons of the screenshots)



GeForce 7900 GX2 4xAA




GeForce 7900 GX2 8xSLIAA


Take a look at the very top of the tree. 8xAA does a slightly better job of clearing up the jaggies on the top branches, particularly the top branch on the left. The foliage in the tree looks sharper under 8xAA as well. You’re going to want to download these screenshots and compare directly to see that though.

Another area where you can spot some differences is just to the right of the tree, along the edge of the building just behind it:



GeForce 7900 GX2 4xAA




GeForce 7900 GX2 8xSLIAA


Here the difference is a little more subtle, but the top edge of the building is ever so slightly sharper with 8xSLI-AA. Honestly though it’s a little easier to eyeball the differences up and running inside the game rather than looking at static screenshots.

4xAA vs 8xSLI-AA Performance

Remember how we mentioned on the previous page that Quad SLI isn’t ideal if you only plan on running 4xAA/16xAF? Now we’ll show you benchmarks as proof. On the next few pages we’ll compare the Alienware ALX Quad SLI rig (which runs with a pair of GeForce 7900 GX2 cards) with similarly-equipped GeForce 7900 GTX SLI and Radeon X1900 XTX CrossFire setups.

First let’s look at the system’s we’re testing…