I agree with an HD 7770 for a minimum with those games. However, as has been said you will likely need a new power supply anyway making something stronger more advisable. What resolution do you play at? At 1920 x 1080 I would say an HD 7850 would be your best bet for the money with an HD 7870 an even better choice if you can fit it in your budget.
Here is an excellent article Tom's does every month on the best cards for the money.