I think the X2 will actually be faster than 2x4870's in Xfire, mainly because if ATI's new "hub" is what I think it is and the xfire side port built into the gpu core is what I think it is then the x2 should scale extremely well, infact I wouldnt be shocked if the x2 actually behaved like 1 large gpu rather than 2, also looks like shared memory could be on the cards which again is another huge bonus.