You can buy a graphics card, but it must be PCI. Not AGP, or PCI Express because those arn't compatible with your system. I had a 3000 with this delema, because PCI cards are outdated, I upgraded the motherboard with the Dimension 4600 motherboard that supports AGP. For performance I would recommend this, but if you are just looking for a little more kick then the integrated graphics, you can try a PCI card.

Last I checked, the Nvidia Geforce 6200 PCI was the best PCI based card. I also liked the radeon 9250, but there have been reported problems with it. If you can track down a 6200 PCI, thats pretty much the best you can get.