I recently upgraded from an i7 920 to an i7 5820k (nehalem to haswell, same as you're considering) and while I definitely noticed an improvement, it wasn't anything earth shattering. I would definitely consider upgrading your graphics first and then later upgrading your cpu, as you'll notice more of an immediate improvement that way. You should also consider a 970 instead of a 770, as it has quite a few new features over the 770 and outperforms a 780. Good luck on making your choice!