You can't upgrade the power supply of a laptop. The external graphic card comes with its own.
Be sure your external dock can connect with a fast enough port on your laptop.
For example, see https://www.pcworld.com/article/2984716/how-to-transform-your-laptop-into-a-gaming-powerhouse-with-an-external-graphics-card.html
Hi so I have a asus laptop and I just started getting into games a lil more on my laptop but I would like to run them better and I have been looking at external graphics cards but I don't know what to get that will be able to run games better that what I have(mainly want gta5 to run on higher settings)but I also don't know what power supply I would need and whatever else there is that I would have to buy so if someone could help he get a whole setup with like what graphics card and power supply I would appreciate it I was looking at the asus strix r7 370 4g and a 500 watt power supply but I don't know what is good the prossesor in the computer right now is a amd a10 7400 Radeon r6 10 compute cores 4c+6g 2.50 ghz