780ti. 700 dollars for one of the highest clocked versions out there: http://pcpartpicker.com/part/evga-video-card-03gp42883kr
3gb of vram so that's a nice perk too. Could get one of the non-reference 290x cards too, but the price is up in the air and they generally fall behind the 780ti in most benchmarks. However, the 4gb of vram might be a larger issue in a couple of years. I wouldn't get the 690 or Titan now, and instead buy one of the other cards I mentioned with the possible intent of going with a dual-gpu setup later if you want.