Do you play at 120hz? Or do you play with high resolutions/multi monitors?
If you play at high refresh rates, I would advice you to go with a 660ti crossfire that gives performance of over a titan and you save a lot of money instead of buying a 780.
If you play multi monitor or something like 2560x1600 then you can get a 780 if you want but it's not really advisable to throw that much money on just for a graphics card.
Spend another $300 on the 660ti, crossfire it, Saving you $400. If you want to upgrade, you should wait for the next generation after these gk110 and gk104 gpu's phase out.
edit: Sorry, For the times i said Crossfire, I mean SLI. I've been working on my crossfire sooo i forgot what SLI was