3870x2 is less than a 4870. A 5770 is, for all intents and purposes, the same performance level as a 4870.
Don't buy anything less than a 5770, but for a significant boost in performance, a 5830, GTX465, or 5850 are you best options. IF you have a system that won't limit your graphics too much (aka bottleneck).
Please post the rest of the specs of your system, to be sure we can recommend something that you can utilize for the money.
EDIT: I noticed your other thread where you mentioned your specs (stock Q6600, 450w PSU) and would probably not recommend anything higher than a 5770, due to both your CPU limitation and PSU. If you were willing to replace the PSU and OC the Q6600, then you could make sure of something stronger than a 5770.