R3SISTOR

Great comparison Here. Overclocking will yield almost identical numbers as well. No need to upgrade your CPU in my opinion. However, a 780 is still a great GPU. But I guess a 1070 is an upgrade

The 1070 will maintain 60 FPS minimum on Max or Near Max settings (Some settings like advanced settings on GTA V are too insane) at 1080p with your current processor.
 
The 1070 + 2600k should be able to keep your rig entirely functional and even near max for 1080p60fps for a good 3 to 4 years more or less. At that point you should upgrade the entire rig but for what you're requesting a GPU only upgrade will do fine.

I found this video: 
 
As you can see, the 2600k falls behind more than others in some games, but for the most part it keeps up. This is on stock speeds. Overclocking yields near identical fps for all processors.
 
Your biggest limiting factor IMO is the 3gigs of VRAM in the 780.

2600k still has plenty of life in it  . Upgrade your gpu instead.

I was talking 3 to 4 years into the future though. I believe that if you plan on such a long upgrade cycle you need to plan ahead and buy a GPU that's a bit overpowered for what you're doing today.

Upgrade gpu first. Unless you are having a bottleneck issue with your cpu then you would upgrade that.

I would just stick a 1070 in there and call it a day. 

Here are two pictures of my CoolerMaster Storm QuickFire TK Blue. I'd been using my Logitech G11 since the middle of 2007 and I decided it was finally time to get a mechanical keyboard, so I chose this one and I love it. The pictures probably don't do the LED lighting justice, it is absolutely beautiful, especially in the dark.

Actually crying right now. Thanks for your support guys. 
 
I've missed my baby.