I've come across a bit of a treat. In a couple months I'll likely be getting a free i7-6700K. My current setup is a 3570K @ 4.6ghz, nvidia 660 (long in the tooth now), 16gb ram.
I'm happy with my 3570K, honestly, it's perfectly fine and I've had no intentions to change it out. I have been wanting to upgrade my GPU though, likely before the end of the year. I've been eyeing a 1060.
That said, I have two options.
1) Sell my 3570K, board, DDR3 ram, to pay for a Z170+DDR4 to support the new CPU
2) Sell the i7, use the money to buy a new GPU, likely a 1060 and then pocket the rest. (My PSU is fine)
I feel like the 6700K is just not a performance upgrade worth it, but at the same time new hardware is always nice. I'm leaning towards #2.