You might not like my answer, but here it is. That thing is OOOOOOOOOOOOOOOLD.
It only uses AGP 4x? Well the best I can suggest would be any Radeon HD 4650 that supports 4x AGP. As for your computer, uATX? That is like Micro ATX but the card is for full tower cases not micro. Still check your PSU to see if it can provide this much power for this one I found.http://www.microcenter.com/single_product_results.phtml?product_id=0326494