Well, in this day in age, for most games, you'd have to go for a card within the range of the Radeon HD 7970 or GTX 750, both being excellent upper-middle range cards.
If you want to go all out, however, I'd upgrade my power supply and get a Radeon R9 290X or GTX 780 Ti. List your power supply and I can get a bearing on what cards would be good for you!