At the moment the Radeon 7770 is incredibly bad value over the 6850, 6790 and 6770, not to mention the GTX 460. At this very moment the best option in your position is the AMD Radeon 6850, or NVidia GTX 460, or if you are looking for something a bit more powerful ... the Radeon 6950. Just get a add in cooler such as ASUS direct CU or GIGABYTE windforce for quieter operation.
Since you already own a 4850 (which really isn't too bad still) I'd recommend that you wait for the new AMD 7800 series to come out in about a month's time. A 7850 will offer better performance and lower power consumption. Hopefully aswel by the time that it is released the 28nm process will be a bit more mature and the price on the cards will drop similar to the prices of the current generations