although, comparing a R700 (aka 4870 X2) with a GTX 280 is apples to apples in that both are on a single pcie slot, the fact that they don't compare it to GTX280 in SLI is a little misleading.
This review has the GTX280 in SLI as well. Still, the 4870 X2 is freaking amazing, especially when considering its multi-monitor support.
http://www.techreport.com/articles.x/15105"The reality is that a single Radeon HD 4870 GPU is nearly fast enough to keep pace with the GeForce GTX 280. Even if you're running a game that lacks a driver profile or simply doesn't scale well with more than one GPU, the 4870 X2 ought to perform awfully well. And when it does get both GPUs going, as our results show, it's by far the fastest single video card we've ever tested. If this is how AMD rolls, it's hard to complain."