Assuming you didn't want to wait for the 3XXX series chips, the 2600 and 2600X should perform relatively close to each other although you will see slightly higher clock speeds with the 2600X and may be able to overclock it farther thanks to the higher end stock cooler (assuming you were to use the stock cooler).
With regards to the RAM, for streaming and even just to play more demanding games having 16gb is a good quantity. I would recommend you get a kit that is at least 3000MHz or higher since Ryzen takes advantage (and performs better) on faster memory, just make sure the kit you get is on the QVL for your motherboard.
For a GPU, I'm quite a fan of the GTX 1060 6GB model right now since it provides good performance, although, if you have the budget, a 1070 or 1070 ti will perform much better.
As a general rule I like to stick to Asus for motherboards, however, I've recently done a few builds using Gigabyte and Asrock boards and have been satisfied especially since those brands tend to have better budget offerings compared to Asus. I will say however, it's hard to beat the Asus BIOS and I have yet to find a brand with one better than Asus.