Good question. Many believe the HD5770 will be brought up as either the HD6670 or HD6770, with a few minor enhancements or just simply renamed (hopefully not!).
Others believe that in addition to it being dubbed the HD6670, the HD6770 will be the world's first 28-nm class GPU, just like with the HD4770 (being the first 40nm GPU). The problem is that the HD67XX is slated for a Q1 release in 2011, much too early for 28nm.
The replacement is already out. It's called 6870. 6850 is replacing 5750. They jumped up a number.
This is true model number wise. But performance wise we are yet to see a HD6xxx card in the same or a little bit higher segment than than HD5770. Which i think Sahdow187 has already mentioned that the 6670/6770 may come in the place of 5770..