A:AnswerIf you are simply looking to add display capacity, find a GT 710. They are cheap and basic. But, they are still Nvidia. Don't mix AMD and Nvidia in a system.
A:AnswerNo, you need a better PSU my recommendation would be more than 450 Watts. If you look up RX 570 it will tell you that it would need a minimum of 450 Watts for your average system. Also RX 570 is equivalent to the GTX 1060 6GB if you would prefer a Nvidia option.