Obviously, as per our conversation, I think this is a beautiful build. Although, $6500?
I finished mine for about $3700(ish), and 2 680's out-perform a single Titan. But, again, great work on it otherwise!
Edit: of course, there are always those other variables. (Titan doing better on multi-monitor setups with the amount of VRAM. But, once you learn things like custom SLI profiles, as well as using Nvidia Inspector, things can go well in favor of an SLI setup. That being said, Some games just won't do well with 2 GPU's, in some cases: a single 680 is better than two (let alone a single Titan).