Wanting/wondering what the upgrade options are. Currently thinking about adding a second, identical GPU. OR upgrading to the current gen card.
I'm not sure you need another/better GPU setup as the 680 is still a top notch card. If you have money burning a hole in your pocket, PM me and I'll give you an address to send the check.
To answer your question, I would suggest the second 680: a 690 is about 5-10% slower than SLI'd 680s; a Titan is about 5-10% slower than a 690; and a 780 is about 15% slower than a Titan.
690/Titan = $1000
780 = $650
680 = $450
Hope this helps.