If you're planning on playing newer games such as Metro LL / BF4 / GTA V ect ect You're going to require a GTX 760, or Readon HD 7950.
However if You're happy with approx 40-45 FPS Average a 7870 would probably be quite fine.
Or alternatively you could wait to see what the HD 9xxx series dishes out and get the lower end GPU that they release.