You're looking at about 20 - 25% better performance with the 680 vs the 660ti.
I wouldn't spent $300 for such a small upgrade If you were able to sell the 660ti for, say, $150 - $200 then maybe but I personally would still find it difficult to justify the money for such a small upgrade. --- Have you accepted Raspberyl as your loli and savior?
Get the 770 if you're willing to spend that much on a new card. It outperforms the 680 by a bit and comes with three free games. --- R.I.P. Mewtwo (1996 - 2013) i7-2670qm @ 2.2ghz | 6GB DDR3 | 1GB Geforce GT 540m