system
A chat on subreddit /r/pcmasterrace
human
Literally the whole computer needs an upgrade. 

Just build new from scratch.
gpt
People are spoiled these days, while you are correct that the whole thing needs an upgrade, that doesn't mean that his pc is trash, maybe all he wants is 60+ fps in some competitve games like fortnite, apex, pubg, csgo, valorant, dota, lol etc. Or maybe he simply doesn't have the money for the whole thing at once and needs to go step by step. I haven't bought a "new" PC since 2008, only upgraded parts one by one and I currently have a config similar to yours.

OP, the answer you are looking for: the GPU, that's your quickest way to better gaming in your current situation, since your current GPU is really bad. The next step would be cpu + motherboard + ram, but you will have to buy those 3 at the same time since yours are very old, don't worry about the GPU compatibility, every GPU fits in every motherboard.

Last thing, since your current GPU is very weak, your power supply might also be weak and you won't be able to power your new GPU, check that before buying the GPU, you might have to get a power supply as well.