I think you would notice a nice improvement if you looked into the X1650Pro for about $125.
http://www.newegg.com/Product/Product.asp?Item=N82E16814102057
Or the 7600GT for about $190:
http://www.newegg.com/Product/Product.asp?Item=N82E16814150210
One thing I would think about before buying: If you are going to do a larger scale upgrade, you will probably end up with a PCIe board, so remember that you will have to get a new video card as well
http://www.newegg.com/Product/Product.asp?Item=N82E16814102057
Or the 7600GT for about $190:
http://www.newegg.com/Product/Product.asp?Item=N82E16814150210
One thing I would think about before buying: If you are going to do a larger scale upgrade, you will probably end up with a PCIe board, so remember that you will have to get a new video card as well