+1 to the 6770. It's in the lower $100s range.
If you want something around $100 or less, an HD 6750 is a great option as well.
EDIT: As AidanJC mentioned, PSU is also something to consider. If your PSU is 400W or so, go for the HD 6770. If not, and you have lets say 300W, an HD 6670 is also a decent card.