Only my opinion but since not all games support sli,such as C.O.D.4, i would go for the 4800 series.So in an unsupported sli mode the ATI would obviously outperform the two 8800gt's.
Wolud it beat two 8800gt's running in sli mode with a compatible game? Not sure about that.Think we will need someone elses input on that,although the 4000 series are excellent cards.
The other thing is that if the same board is being supplied i would imagine it wont be a crossfire board as i think they have to be either one or the other.
This will mean that in the future, if you wish to add a second ati in crossfire mode you won't be able to.