system
A chat on subreddit /r/pcmasterrace
human
if you arent going to buy a new computer, ssd, ram another matching 8gb. then graphics card. then save up for a brand new system.
gpt
This. An SSD first, matching stick of ram could be good if you're running newer stuff (or any web browser...).

But really, there's no single bottleneck in your system. Money spent upgrading it will very quickly be diminishing returns except for the SSD. Look into buying a new system before you polish the turd. Used hardware is perfectly acceptable to save money, just make sure you can test it first.