I just played this game, 2x 5850 is a fps monster but microstutter is a game killer for me, half the frame rate on a 5870 in crysis looks and plays so much smoother than micro stutter crossfire setup. I had 2 systems setup side by side to test this, its real and ruins the higher frame rate you get.
5870, then wait for the 5870 to drop in price later on to buy another for the same you'd spend on two 5850s right now. The 5870 is a beast of a card and can handle just about every game under the sun right now, and it's good to have the extra slot open when you actually need it.