Yes, you definitely need one if you're streaming video games. The GPU to get will highly depend on what you want to stream though. Say if you're gonna stream CS:GO or another non-graphic intensive game, then it wouldn't really make sense to get a GTX 1080 Ti unless of course, you game a lot on the side, then that might be worth the investment.
I suggested 16 GBs of RAM because Ryzen works best with a set of 8GBs in order to maximize its output (3200 ones that are "Ryzen" compatible) and anyway, more RAM = better multitasking. I really wouldn't recommend 8 GBs of RAM to anyone building a gaming/streaming PC, maybe for regular multitasking, you could get by with just 4-8 GBs.