as we have a multi post ill copy and paste my answer across as ether or will be deleted.
the appalling information given by HP doesnt even detail what kind of pci version it has.
Its definitely pci e and 16x lol, id hazard a guess at 2.0.
Now that doesnt hugely matter as PCI is backwards compatible, and the difference between a pci 2.1 and 2.0 is minimal, the speed tranfers are still the same.
Im going to be blunt and say you cant really upgrade your GPU to anything half decent with your current power supply. As its only 300w it is less than most modern cards draw.
a 500w+ is recommended.
so you need to factor in the price of a branded PSU before you look into getting a new card.