Here. i5 has best price/ performance on the upper end, and the 7870 xt is a 79xx with a slightly lower core count and clock. If you are adept at overclocking, you could push a 7870 xt to 7950 performance. Case and Psu are a little pricey for a $700 build, but nice to have(don't go cheap on a psu). If you want to, spend a little more on the mobo, although that b75 msi should work fine.
Note: if you need a os, drop to a 7850(don't go lower, be more frugal on the case), and pick up windows for $90.