Assuming the pcie slot is x16...
http://www.newegg.com/Product/Product.asp?Item=N82E16814102022
Radeon x1900gt 256MB
http://www.newegg.com/Product/Product.asp?Item=N82E16814161045
Radeon x1650PRO 512MB
http://www.newegg.com/Product/Product.asp?Item=N82E16814122010
GeForce 7900GS 256MB
http://www.newegg.com/Product/Product.asp?Item=N82E16814130021
GeForce 7600GS 512MB

Of course there are some decent ones under that price range, so just look around to see what there is.