From what I've been able to determine, your system does not have an AGP or PCI-E port for a graphics card. You would need to use a PCI graphics card (note: there is a difference between PCI and PCI-E and the two are not compatible).
In your case, I would probably recommend a Geforce 8400GS like
one of these. Not too pricey for a low end card.
-Wolf sends