An overclocked 2600K is going to be just about the best performance you can get, so it won't be bottlenecked any more than the latest and best overclocked processor. Hopefully, you are running multiple monitors and/or higher resolutions, because that will likely prove to be your bottleneck if you are on a single 1080 screen.
Here are a couple reviews. Note that Quad-SLI is officially not supported by Nvidia, but still works all the same.
http://www.vortez.net/articles_pages/gtx680_quad_sli_vs_hd7970_quad_crossfirex,11.html
http://www.guru3d.com/articles-pages/geforce-gtx-680-3-way-sli-review,1.html