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".