The GTX 770 is an overclocked GTX 680 that makes use of GPU boost 2.0. It is cheaper than a GTX 680 and from what I have seen has good overclocking potential. The GTX 780 is a Titan light. I personally upgraded from a GTX 670 ftw to an Stock Cooler GTX 780. I did this because I use a 2560x1440 monitor. While my GTX 670 ran the monitor very well there were a few games than pushed the limits of the available vram on my GTX 670. In comparison to my GTX 670 my GTX 780 does show a rather healthy improvement in some of the more demanding games I liked to play.
If you plan on getting a 2560x1440 or surround setup I think the GTX 780 is a worth while investment and will last you a long while. If you only want a 1080p monitor a GTX 770 would fit the bill. My GTX 670 was overkill for most games are 1080p.