As Deus said, you already have a powerful set-up and grabbing another 680 would be the obvious solution but if you are worried about micro-stuttering and other issues that SLI can bring I would look into getting an EVGA GTX 780 SC w/ ACX cooler:
http://au.evga.com/products/moreInfo.asp?pn=03G-P4-2784-KR&family=GeForce%20700%20Series%20Family&sw=
This card is shown to be approx. 5% slower than a TITAN and for $350 less! It is the best bang for buck, it is made by EVGA who are known to have the best customer service for their products and the bearings in the fans have a 12 year life span.
EVGA did not pay me to do this btw for all you trolls, I simply admire this card and I also wish to buy one in the near future hopefully.
Cheers,
Matt
P.S. If you have any questions, don't hesitate to contact me!