When buying AMD cards with the current numbering scheme, follow these basic rules:
#1. Don't buy any AMD card where the 2ND number is less than 7.
#2. If you have to settle for a card where the 2ND number is a 7, make sure the 3RD number is also at least a 7.
#3. If at all possible, try to get a card where the 2ND number is an 8, AND that it is followed by at least a 5.
so, as an example, here's some bad cards:
6450, 6570, 6670, 6750
here's an example of good cards:
6850, 6870, 6950, 6970
the 6770 is borderline. i'd spend an extra $25-$40 and get the 6850 which is way way better.