Depending on if you are looking to go with nVidia or ATi and if you are willing to spend that kind of money on an AGP video card, you should get:
[link=http://www.newegg.com/Product/Product.aspx?Item=N82E16814150223]Geforce 7950 GT 512MB[/link]
or
[link=http://www.newegg.com/Product/Product.aspx?Item=N82E16814241049]Radeon X1950 XT 256MB[/link]
Those were the two fastest cards I could find on [link=http://www.newegg.com/Store/SubCategory.aspx?SubCategory=48&name=Video-Cards]Newegg.com[/link]
P.S. Performance-wise, these two cards are practically neck and neck.