Here are a couple of sites looking at what you get with nVidia's top of the line graphics in SLi. First off, we've got the guys at the [H], who put a pair of 8800GTX's in two different setups: one with an AMD chip, & the other with an Intel chip. They then game the hell out of both rigs, & tell you which CPU offered the better gaming experience. Only those with deep pockets need apply ;)

Then we've got the guys down in the Damage Labs, who throw a bevvy of 8800 cards into a rig to see how it flies. They include both GTX & GTS varieties in their testing, as well as a comparison to an X1950XTX CRossFire-based rig, & 7900GTX SLi for good measure. They found some bugs with the latest official nVidia drivers, so they also run some beta drivers to see if there'd be any improvement.