For such an old system, your DE matters more than anything. I’d recommend XFCE, or even LXQT. Definitely upgrade your RAM though, that’ll give it a nice boost - DDR3 should be pretty cheap now. Any cheap SSD should be a big improvement as well.
I’ve got Zorin installed on my mum’s PC (which is just as old as yours), and it works really well, even on a regular HDD.