Cards (single card) with similar performance, GTX580, HD6990, HD7970
Cards (single card) with better performance, GTX680, GTX690, and HD7990 (Not released).
---- The above statements only take in consideration of 2650p or 5760 (Tri-monitor). Still somewhat true in 1080p ----
For a substantial upgrade, it would definitely be GTX680. However, GTX670 OC is also very very tempting.
GTX680s on newegg. Since you are willing to pay for a nice one, I would suggest one with an aftermarket cooler.http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&N=-1&isNodeId=1&Description=GTX680&x=0&y=0
GTX670s on newegg. If you are comfortable on OCing it. (The performance is similar after just a little OC...)