A few things:
You have a GTX 970. If you are located in the US then please submit your claim for $30, in reference to the GTX 970 settlement. Palit is one of the AIC partners listed as a plaintiff:
https://www.gtx970settlement.com/Home.aspx
I approximate that your i7-2600 is equivalent to the processing power of an i5-6500, based on architecture, base and boost clocks.
If I were in your position, I would do the following:
Purchase the GTX 1070 and install in your Z68 / i7-2600 setup.
See how the performance is.
Fallout 4 is definitely one of those titles that takes advantage of a current generation i7, therefore you would most likely see a appreciable improvement with upgrading to a i7-6700K. I love the idea of keeping the motherboard and upgrade to a i7-3770K, however I see those CPUs selling used on Ebay for $225 - $250. For that price, I'd rather upgrade to a new platform. Since your current one is 5 1/2+ years old, I'm assuming that you tend to keep CPUs for a while before upgrading. Nothing wrong with that.
If your current CPU performance is good after being paired-up with the GTX 1070, then perhaps wait for Intel's "tock" (Intel's tick-tick-tock cadence). kaby lake is a tick, and the processor that release one year later (Q4? 2017) will be the tock.
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submit GTX 970 claim.
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buy GTX 1070.
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buy future Intel 10nm chipset motherboard / CPU / memory (or Z170/200 series 14nm chipset components now).
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buy Windows OEM license from Kinguin for ~$25 - $30. Swapping motherboard may necessitate new Windows key.
! kaby lake possibly may require Windows 10 to be fully supported.
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sell GTX 970; sells on Ebay for $175 - $200.
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sell i7-2600; sells on Ebay for $125 - $150.
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sell motherboard; sells on Ebay for about $75.
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sell memory.
Upgrading the CPU alone is obviously a simpler solution. I'm just giving you my perspective.