isnt nvidia gonna launch new product on ces 2018 in january ?I don't think the core i5 4570 will bottleneck a GTX1070.
I'm running a GTX1070 with a Core i5 3330 and it runs great even at 4K in older games.
If I were you, I'd get the GTX 1070 first. Then if you feel the CPU is a bottleneck then you can upgrade it.