Or, if you don't want to deal with MIR, Radeon 7750 like mohit9206 and lchrisk said.
It's a shame you can't even get a GTX 550 Ti for $100. Not that it beats or even comes close to the 7770, sad. You can get a GTX 650 for $100 but the 7770 beats that too. And everyone wonders why we don't recommend Nvidia.