Right now the R7 260x is the best performing card in your price range. I suppose you can attempt to overclock one for slightly better performance.
Keep in mind that the benchmarks I provided a link to are for max / ultra settings / very high / uber or whatever the highest possible settings in the game is called. Simply dropping down to high can improve performance.
The following review have benchmarks of games for both ultra (or whatever...) and medium settings which you look at to help you gauge what performance you might get setting graphics quality between those two settings. However, I must stress that the test rig uses an Intel Core i7 3960X (Sandy Bridge-E) CPU which has 6 cores. There is no mention of the clock speed so it...