You have an agp x8 slot and want the best upgrade? Well the best (and last) AGP card is the 4670.http://www.newegg.com/Product/Product.aspx?Item=N82E16814161337&cm_re=4670-_-14-161-337-_-Product
You can save a significant amount of money by getting the 4650 if the 4670 is out of your price range.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102851&cm_re=4650-_-14-102-851-_-Product
Neither of these are amazing, and I advise you to just save your money and get a new computer. However if thats not possible those are your two best options.
However, if you can can you tell us the motherboard, cpu, ram, and resolution you use on the computer? As well as your PSU?
If you don't know them you can downoad CPU-z to tell you the CPU, ram, and motherboard you use. You will have to open the case to find your PSU.