If you use your computer for gaming mainly, the biggest performance increase will be seen with a GPU upgrade for sure!
The 770/780 will run circles around your old GPU, and who knows if you ever need it, just slap a cooler on your i7 and overclock it a bit!
Overall the i7 2600k i still a very capable CPU, and you have the 'k' version which allows for some overclocking with an aftermarket cooler. The overall performance increases of GPU's since your 570 have been comparably higher, and thus you will see a bigger effect on gameplay experience ( smoothness/FPS/how high you can crank your settings ) when you upgrade your GPU first!
Also seeying as Haswell uses a new socket, you'd have to replace your motherboard etc too. So it's convenient too
Goodluck!