Considering a 670 is within 6% of the performance of a 680, you'd see little gain just upgrading to a 680. Very very little. If you really need the performance, SLI your 670. Although if you are just gaming on a 1080p monitor, it will be overkill.
You know ive heard people say that one card albeit a 670 680 is enough but these peeps aparently not gamers cause i seen my buds 3930 with 2x 680 s only get 45 to 50 fps in the new gw2 in certain wvw and pvp instances it drops evenn lower sometimes
. And the same goes for wow set to highest..