His motherboard only has AGP and PCI.......So obviously PCI Express will not work.http://www.newegg.com/Product/Product.aspx?Item=N82E16814133245
AGP has greated bandwidth than PCI so an AGP card is better. You can get a HD4670 or 4650 AGP although they are more expensive than PCI Express.
Between 6200 AGP or 8400GS PCI I would go for the newer card. The 6200 is really bad and the 8400 PCI will probably be faster even though its on a slower bus.