Well a lot of the decision has to do with what AMD CPU you're running, and the Games you want to play, and the resolution you're playing them on i.e. monitor. Also, if your current GPU is capable of Crossfire then that might be something to ponder -- to a point on the second x4 PCIe slot.
Keep in mind most expensive GPUs will be 'bottle-necked' to your CPU in comparison to a Sandy Bride.
For $170 XFX HD-687A-ZNFC Radeon HD 6870 -> http://www.newegg.com/Product/Product.aspx?Item=N82E16814150506
Lifetime Limited warranty.