There is also the option of getting a 6950 2GB. It's $229 + Free Shipping / Before $30 Rebate. Even this is barely a significant performance gain; maybe 25% in best case scenario in specific games. Your current 5850 as it's still a very capable card and can run most modern games at high/medium settings up to 1080p. I would recommend you to wait about a month or two until Nvidia and AMD release more of their mid-range cards . Sorry to disappoint you but until Nvidia and AMD offers more options and possibly drop prices, there is no worthy upgrade at the $200 mark.
5850 vs 6950 2GBhttp://www.anandtech.com/bench/Product/512?vs=510