What is your budget? You could probably get a 1080 and it would probably be totally fine (and totally overkill) for pretty much any game. only in a handful of games would you really stand a chance of bottlenecking your GPU (but that will depend on just how hard you're overclocking that CPU of yours), but that isn't the end of the world a; a 1080 operating at 80-90% is still going to decimate anything you throw its way at 1080p-1440p. You could get a 1080 now, and then upgrade to Kaby Lake (intel's new CPU lineup) coming out in the fall. or if you find that you're not having any performance issues at all. If not, you could always wait for Zen or Cannonlake.
Bottom line is, if you're going from a 670 to a 1080, it is going to feel like a brand new machine anyway. that is a SERIOUS performance jump. What resolution are you planning to play at?