Upgrading my computer to give spare parts to a family member

[WestTexan]

Current build :

Ryzen 5 3400G

RX 570 (4gb)

2x 8GB 3200mhz DDR4

MSI B450 Tomahawk Super Flower SF-650F14EG(BK) Leadex II 650W, this is too much watt for my build. right?

 

I plan to upgrade every single component over some time then give this current computer to my younger brother but currently I am on the fence as I do not know how much money I need to spend. First I'd like to upgrade my cpu then gpu and go from there to replace everything so I will be staying with Ryzen CPUs thats compatible with my B450 tomahawk

 

Here is what I want : an 8 core/16 thread cpu, a friend recommended the Ryzen 5700G.I'm not sure about the GPU though. I definitely want a Nvidia GPU but if an AMD counterpart is better/cheaper then I will go with that. For the GPU I'd like to spend 300 USD - 400 USD. I am not sure if GPUs under 300 USD are worth the upgrade

 

I'd also like to know what type of PSU ( and its Wattage) I'd need with a system with the CPU/GPU as stated above.