I'd get two HD6870s. You would have to spend around $550 though, as the 6870 is $250. Alternatively two DirectCU 6850s will overclock like a beast and you'll have around $70 to spare.
Alternatively if you don't want to overclock you can try for 2x GTX460's. The FTW edition is around $240 though (IIRC), and a 6870 is a better card.