There's no telling if the quad crossfire scaling will be better with the 4870x2 than it is for the 3870x2. We can only wait and see.
Also, if you don't overclock that CPU pretty hard, it's likely to bottleneck these extremely high-end graphics solutions.
Well, they claim the 4870x2 does not use a bridge chip, shares ram and act as a single gpu. If this is true, then it would scale as dual cf, and not quad cf, and thus perform far better than gtx280 sli. ATI/AMD may be blowing smoke though.