Graphics wise you cant do to much with $200 considering you allready have a 4850. What are your computers specs? CPU Mobo PSU? and screen resolution. Depending on where you live you could get another 4850 for around $110 shipped and that would nearly double your FPS. If you could save up to $300 for a 5850 you could gain DX11 and run any game at max on 1080 screen.
Oh you want to buy two, that changes things a little. Performance wise two 4850 for $200 cant be beat however they dont have DX11, which will give you some future proofing for later. For DX11 and $200 a 5830 would be great however I do not know when they are said to be released, it should be as powerfull as a 4890. And for comparison that 5850 is as good as two 4850s in crossfire however it is $100 more expensive.
Basically it comes down to "better performance for less money now and upgrade sooner" or "spend alittle more now but enjoy it longer".