This is wrong section, but anyways.
He has a 775 socket CPU.
There aren't many choices to go with that wouldn't hold back a 500 series GPU. That being said, your choices are between:
Intel Core 2 Quad Q9650 Yorkfield 3.0GHz
Intel Core 2 Quad Q9550 Yorkfield 2.83GHz
Or anything on this page, if you don't like the ones above. http://www.newegg.com/Product/Produc...CH&Pagesize=20