I wanted to add, that Crossfire works (incase people misunderstood me) even with the 512 4870, but you'll restrict even the 4870 X2 to using only half of its ram, which pretty much cripples the setup in comparison to the results I saw the 1 gig 4870 ( how much more it gained).
I would stick to just 2 4870 512s if thats the case.