70 FPS in what game? By "best graphics" do you mean higher settings or just the best video card for the price? The systems forum has a lot of system examples at all sorts of prices, including the sticky posts with voting for best system for a budget.
http://www.tomshardware.com/forum/forum-31.html
For your price, I would say an nVidia 1060 6 GB or 1070 would be good. A GTX 1070 is about $400 which leaves $600-800 for the rest of the system, plenty as long as you don't go after a very high end motherboard or case. If you want to minimize costs, an RX 580 will get you high FPS at 1080 resolution in any game and will be about $100+ cheaper than a 1070. If you want the system to last a bit longer at higher quality and FPS, then a 1070 would be good.