I wouldn't really upgrade anything on that, one important thing that everyone forgets to mention is their monitor setup, though i assume you're using a 1080p/60hz one.
The i7 is good, overclock it (if you haven't already) since that will benefit performance a bit. The i7 4790K which is the newer version requires a new mobo and is basically almost only a clock increase compared to yours (both on 4.4Ghz and performance would be almost the same), so it would be stupid to upgrade to that.
ONLY real thing to upgrade here would be the GPU, but it's not even close to the level where i would do an upgrade really.. 770 will do gaming in high or more settings in any title, there's no reason to replace it for a GTX780 or better, UNLESS you have something else than a 1080p monitor etc. 120/144hz monitor or 1440p monitor.
So keep your system like it is now, rather spend the money later when the difference wont be so little.