For dota 2 at max, you'll only need at most a gtx 660 or 7850. You might even be able to get away with a 650ti or 7790, not sure if it'll cause a reduced FPS though, and if it does, it would be an extremely small dip in the fps. Probably not even noticeable, like, 50 fps or so.
You should not only play Dota. Play other games as well. The processor is pretty strong, it can take up even the highest end single-die card such as the 780 Ti and the 290X played at 2560x1440. I recommend this: