Depends on the title, as titles utilizing <5 threads, still leaves plenty for streaming. Some newer titles are using more, and that's likely to become the 'norm' over time (although that's been said for years now)....
Personally, I'd hold out for CoffeeLake availability in a couple of week..... you're going to be looking at 6core+ chips in the 'mainstream', without having to invest in an "X" platform, where motherboards are usually $200+.
I'd expect Z370 to be priced in the same (rough ballpark) as Z270 ($100-$150) with CoffeeLake prices similarly to KabyLake.
Alternatively, Ryzen could present a solid value for money option for you here..... a Ryzen5 1600 + B350 board (for example) would net you (relatively) similar performance*, for about $250. The 7800X is what? $350 alone?
*With new updates, Ryzen is closing the "gaming" gap with Intel a bit, especially when paired with more "modest" GPUs. The gap is still pretty sizeable, due to Intel's higher IPC when paired with "top tier" cards like the 1080TI or Titan.