I am assuming you have an AGP video port in your PC. Therefore, here's a list of AGP video card that's probably compatible with your computer.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709+600007850+600007817&QksAutoSuggestion=&ShowDeactivatedMark=False&Configurator=&IsNodeId=1&Subcategory=48&description=&Ntk=&CFG=&SpeTabStoreType=&AdvancedSearch=1&srchInDesc=
The best bang for the buck would be the Radeon HD 4650 for $80.