Yah, that's a tough decision sitting there.
A 6600K gets you onboard for DDR4 and PCI-e3.1 and such.
A fourth gen 4690K and a decent mid range board will only be 10% behind depending on the GPU.
It really depends on what you are going to use it for. You've been gaming on a laptop? Or is it just
general computing?
I think the most bang for the buck in the case of gaming is to go 980 Ti and build out from that or
a 980 with an eye to SLI in the future to reach for 4K. You can go 144Hz now or 1440p. 1080 monitors
are very affordable so that's yet another data point.
I'd price out a 4th gen i5 with 16GB RAM, a fast SSD, couple of TB's spinner, a middle to high
end motherboard and a 980 or above GPU and compare that to a 6600 setup.