Well, I suppose just about any PCI graphics card would work for you. What to recommend is probably going to depend on what OS you're running. If you're running Windows XP, then I'd probably just recommend the cheapest NVidia Geforce 6200 you can find. If you're running Windows Vista, 7 or 8, then I'd be looking at the Radeon HD5450.
Additionally, if you haven't already, then make sure to upgrade your RAM to the maximum amount of 1GB.