OCC has published a new review on a Crossfire vs SLI Performance Comparison

When you look at the scaling in performance you get from going to a quad GPU setup, it is not going to be anywhere near 100% in most cases. There were a few instances where the scaling on the HD 4850x2 and HD 4870x2 scaled closed to 100% - which was a real surprise. This was the exception, and not the rule. In turn, there were games where there was no scaling whatsoever with cards in CrossFireX, while the SLI combo scaled well. That's something that could be a driver fix, to enable better CrossFire support in games. To use all this video horsepower, you will need a monitor that runs no less than a 1920x1200 resolution, as anything else means you are just throwing money away. The ideal solution is to go with that 30+ inch monitor you have been lusting over to take advantage of the firepower. Currently, Nvidia and its SLI technology is the performance winner here, on this system, with the drivers and video cards tested. The numbers are the numbers, and this is what they show. ATI has great price points, but they just can't deliver overpowering performance for that price. On the other hand, Nvidia delivers the performance but at a steeper price. The old adage "you have to pay to play" comes to mind here.

Crossfire vs SLI Performance Comparison Review