I'd suggest the 6850 over the 6770 because it would be a bigger upgrade. The 6770 is only as fast as the 4870 and won't be much of an upgrade. All the new mid range AMD cards are pretty quiet if you get one with a custom cooler. If you are really sensitive to noise and don't mind paying a premium, the custom 7770 are very low power, low heat and low noise.
I recently upgraded my sapphire 4850 1gb after some researching to a xfx 6850 1gb for $139 off newegg.com im happy with the performance I get for the price I can run BF3 on high settings keeping high 30-40 fps at 1900x1080 and in the 60's fps on medium not bad for under $150.
also keep in mind that if you go with anything greater than a 6850 you would be required to have a power supply with two 6 pin pci-e cables to power it. the 6850 only requires one.
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