Try this test:
Run YOUR games, but lower your resolution and eye candy.
This makes the graphics card loaf a bit.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
Your psu might limit how strong of a graphics card you can use.
Your lack of a UEFI capable bios will restrict you to an older gen card.
Something like a GTX750ti might be a good upgrade, it sells for $70 on ebay.
The good thing about updating a graphics card is that you can use it on a future cpu/mobo/ram upgrade.
If your cpu is lacking, then I would not try to add ram or change out the cpu.
I might guess that you can buy a modern intel or ryzen...