I will bet you got a lot of good usage out of that card. Anyway both cards are so close in what they can do the 6850 shines on some things better and the 7770 shines better as well.
If it were me I would go with the 7770. For one reason it draws less watts then the 6850 so not to stress your PSU and You might not have to upgrade your PSU with as you might with the 6850.
So here is a bench mark the 6850 vs 7770. I hope this helps and good luck to you.http://www.anandtech.com/bench/Product/539?vs=536