If your PC is a bit elderly and you are running a very recent version of Mint, it's possible that it simply doesn't have a driver for your old VGA card, at least not straight out of the box.
It's also possible that there's a problem with the GPU or VRAM, so putting a new card in will address that.
BTW, are you sure about the model of your PC? WS4600 isn't recognised on the HP support website. Is it perhaps an XW4600 Workstation?