560 Ti if you want Nvidia or Radeon 6870 if you want ATI. Both are fairly close in benchmarks and the hands down best cards for price to performance ratio. You'll get a lot of use out of either card for quite some time. Despite the fanboyism you might hear on forums both companies drivers have relative parity these days so my suggestion would be to just grab whatever gets you the better deal on price.
Playing: Bloodborne, Pillars of Eternity, DOTA2 Watching: Vikings, Dig, The Jinx