i read somwhere that the GTX 780 is a GTX titan with less features, and the GTX 760 and 770 are just higher clocked 660 and 670, But if your not in a compulsive buying mood, i think you should wait as either way the price of the 600 series with decline when the 700's come out.
Not to say the 700's wont be amazingly fast cards, by what i found out, your better getting a GTX690.