Dependin on wot mobo you have. If possbile, you can post you mobo on the site and people can help you easily
If you have a ATI PCI Express then you can simply purchase a new card. The 1900 XT / 1900 XTX is the best card of ATI at the moment.
If you dont have a PCI express slot on your mobo, then depending on which chipset you have, you can purchase an AGP card. For your mobo, this ATI card could be fine:
But in my opinion, buy a new PCI_Express mobo then you have plenty of space for upgrading in the future.