Question Is this build good for medium quality streaming and gaming??

Why would you change the GPU? The RX 570 comes with ShadowPlay which is great for recording and streaming. It is also a great 1080p gaming card. If you were wanting to change it, there is nothing in that price range that can even touch it, so you'd be paying a lot more for a meaningful upgrade.

The Ryzen 5 2600 is a good CPU, buuuuuuut the 3rd gen Ryzen products launch on the 7th, so you might want to wait. Also, 8GB isn't a lot of RAM in this day and age. We are at the point where 8GB is the minimum that should be considered.