If it is a pci x16 slot then you can buy like any new graphics card. So you will have tons of options. Look inside the case and compare the top slot (nearest to the cpu) in the PCI/PCI ex row to images of pci express slots on google. Figure out what you have physically, and then we can go from there.
I would think it is an X16 slot, and if it is, what is your budget?