Well first you should take a look at what your motherboard has to offer. This is what I got when I googled your board model number http://www.msi.com/product/mb/K9N6PGM2-V.html.
It appears to be quite old as it only supports up to ddr2 ram as well as the PCI-E slots are gen 1. I would take a look at a cheap GPU such as a nvidia 640 or a radeon 7770 or even lower as your CPU probably cannot handle a high powered GPU.
It may just be time to upgrade the whole PC. The integrated graphics on the new haswell chips is very good at cranking out some decent graphics I have heard.