I would defintely sell the 4870 and get a single 5850.
I've had crossfired 4870's and a Phenom II 550 (unlocked to quad though). At 3.9ghz, I scored 18k with a single 4870, and crossfired I score 20.5k. With a single 5850, you would probably score the same or only slightly less than crossfired 4870's PLUS you will then have DX11.
Yet you said if I got a 5850 I "might" reach your scores