The best you can buy is either a 560/6870. This also depends on your resolution and power supply you have cause both are limiting factors.
+1 - you wont be able to get a crazy good card for that price, but these are top contnders for that range. posting your full specs is a better way of getting closer to what you will need for your system and price range
anything within that price range you wont see much of a difference. Again - posting your system specs will help everyone with their suggestings - the 5770 isnt a PoS it is a pretty capable card for its price range. the price range you are in now will give you a slight upgrade from the 5770. you would be able to afford the 6850, 6790, or a 550 - all of which fall into right around the 5770 is with slight increases in performance. Your system might have somethign else that is restricting the 5770 from reaching its full potential? what resolutions are you playing at?