NVIDIA vs ATI Multiple GPU Scaling Compared in 7 Games
 

In every test but one, GeForce GTX 285 SLI leads the way in performance. Often the GTX 280 SLI pops up next. That's surprising considering the fact that AMD doesn't have as heavy hitting a single GPU part. And it also doesn't take into account the fact that these two solutions often come in very low in the "value" lineup and not that much higher in performance than something like the Radeon HD 4870 multiGPU options or even the GeForce GTX 295.

The cheaper Radeon HD 4870 X2 often does better than the GTX 295, and often multiGPU AMD options have better value than the highest end single GPU options from NVIDIA. But the real stand out has to be the Sapphire Radeon HD 4850 X2 2GB. This unique card really shined and held it's own all the way up to 2560x1600. While a 1GB 4850 might not make much sense (the extra RAM only really helps at resolutions where the 4850 can't keep up in terms of processing power), the 1GB of RAM per GPU on the 4850 X2 2GB really helps make this single card multiGPU option high end.

http://anandtech.com/video/showdoc.aspx?i=3517&p=1