http://www.anandtech.com/bench/product/829?vs=827 This is a comparison in performance. Personally, i wouldnt pay 200$ for this increase in performance, i would rather tweak down graphical settings of the game, but its well known that as you get up the gpu their value decreases (performance per dollar). It also really depends on the game you are playing and the settings you want to play the game on, this answer was assuming you play games as most ppl as bf4, crysis 3 maybe and the demanding game.
EDIT: There is another point, the vram of 780 is 3gb while of 770 is 2gb. Games might be using more vram especially if you are going to game at higher resolution or use intense AA settings. However, this problem can be solved, if you are not willing to spend more than 300$, by buying a hd 7970 ghz edition or r9 280x which have 3gb of vram and have nearly same performance as the 770.
I don't think it is worth spending $200 more and get a bit of performance boost. As you buy better card, their prices get higher even if they are not much better than the other gpus which perform similar or less. If your budget is less than $350 then buy the Sapphire R9 280X a it is quite better than gtx 770. If your budget is less than $300, then Gigabyte R9 280X is the best gpu you can ever get. Unless you wan't gtx 770 badly, go for r9 280x which is equal or mostly better than gtx 770. Not really worth getting some bit of Extra FPS in games by paying $200 more, add them to buy even a stronger cpu, as Games are also becoming cpu intensive. And you will be able to play any game on ultra with gtx 770 and r9 280x, so consider it too. You can even overclock your gpu if you wan't better performance but paying extra $200 is not the best option here.