Considering this, does RX 470 need external power?
AMD Radeon RX 470 - On your average system the card requires you to have a 400 Watt power supply unit as minimum.
Similarly, does RX 560 need external power? Like the GTX 1050 Ti, it depends on which kind of RX 560 you get. For example, this specific model of the RX 560 4GB (and the other two cards below), does not require external power.
Simply so, is RX 460 good for gaming?
The RX 460 will play pretty much anything on medium-ish settings (i.e. Battlefield 1, Titanfall 2, Skyrim Special Edition, Fallout 4, etc.) at 45+ fps. Popular, easy-to-run games such as League of Legends will run at maximum. It's pretty much the best performance per dollar card at the $100 mark.
What power supply do I need for RX 570?
450 Watt