There's so many options. Let's narrow it down to around 100-120 bucks. Thanks guys.
For that price, 6770 is good, if you can go up just a bit more, a 6790 would be a better match for your system. A 6770 will be holding up what your computer can run, 6790 will be closer to the optimum match of components. 6790 is close to 20% faster than a 6770 for not much more money, maybe 10% more.
However, spending the extra $10 for the HD 6850 is more than worth the performance increase. Besides, the HD 6790 and HD 6850 both use about the same amount of watts so might as well go for the more powerful card if you electricity bill is going to end up the same.