Depends on what games you have in mind and how far into the future you want to be able to play without upgrading. You could build an i5/gtx760 system for around $800 and not have to upgrade the 760 for a few years to keep playing on ultra settings, or you could buy a 780ti and not have to upgrade it for the next 6 or so years.
For $700, you can build a good system as long as you don't need things like a monitor, keyboard, mouse and a copy of windows. If you need those things, you will need to increase your budget for the build.