If you are going to run at stock I would suggest an i5 9400. It should run at higher stock speeds then what you have, give you a few extra cores, and save you some hard earned cash. You would only buy those top teir chips for high FPS gaming and if you were planning to overclock where your graphics card was so powerful and spitting frames so fast that the cpu would actually become the bottleneck. Given your situation and your gpu an i5 would be perfect, would not bottleneck you in any way, and most importantly save you a lot of money. It would also allow you to get a cheaper mobo since you don’t need a z series board to overclock. Your current i5 is only a bottleneck now due to the slower clock speeds of the sky lake generation, where really any cofeelake or newer cpu should suit your purpose in that department even out of the box.
If you are planning on overclocking in the future tho to do some experimentation, the 9600k would be a good bet and would be perfect for your situation as well. Those top teir i7s are mostly overkill for anything below a 2080 or 2080ti.
That's exactly what it means.
The 1070 might have a faster clock speed out of the box than a 1070TI, but that doesnt mean its faster, the 1070TI for example has more CUDA Cores. It's like comparing CPU's, higher clocked quad core CPU's lose to slightly lower clocked six core CPU's (ignoring hyper-threading/SMT)
the 1070ti is worth getting over the 1070, however with the 10 series of GPU's, it's not always recommended to buy "OC" editions because the GPU will usually overclock itself with some small tweaks as long as you have a good cooler like the STRIX.