What are you going to be playing?
is it just for gaming or are you going to be doing other things with it?
What resolution are you going to be gaming at? i assume 1080p?
Have you already gotten the rest of the parts for your computer?
If not maybe you an tell us your max budget and we might be able to put together a build list that might be better than what you were originally going to get.
I always suggest getting the highest end GPU you can, so i would suggest getting the 770, if you can sell an arm and a leg, it would be even more worth it to get a 780, its a totally different GPU architecture based on the GK110 processor, which is a monster.
But here are some graphs you can look at that might help you decide.
benchmarks are from guru3d.com
look for the 760 and 770 in each graph and compare them against each other. These benchmarks are in 1920x1200, so thats a slightly higher resolution than 1920x1080 which i assume you are going to be gaming at, so figure in each game you'll get maybe 5fps better performance with each card because of the lower resolution. Remember, if you are using a 60hz monitor, you want the fps to be as close to 60fps as possible.