WEI is not a very useful benchmark. Each of the two video cards can perform slightly better than the other in certain type of games and it also depends on the driver version used. The 6770 has the same shader count as the 4850 but can also support DX11 and has a completely newer architecture. However it is lower on some of the other specs if you compare them on GPUZ. Also, 6770 is essentially a 5770 hardware wise with a couple of more bios features.
Totally agree with this. That is why is it recommended to jump at least 3 levels of upgrade to have decisive improvements in graphics performance.http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html