depends exactly how much you'd like to spend.
$110 HD 7750
$125 HD 7770
$140-$150 HD 6850
$170-$180 HD 6870
$215-$220 GTX 560ti(Some cards,like the galaxy and zotac have a $25-$30 MIR)
$235-$250 HD 7850
$310-$320 HD 7870(some cards like MSI and HIS offer a $20 MIR card)
Have a look at those cards and decide which one you want. Each has its perks. The lower end cards will require less power,but games won't be played on the highest settings. The higher end cards will need a little bit more power,are a little bigger but will give you a great gaming experience.
Hope I helped,