The best cpu for this board would be a Core 2 Duo Extreme... But depending on how much you want to spend on one, just a regular Core 2 Duo would be fine. Newegg has two referbished cpus for 52 - 56. Elsewhere may have more for different prices, but because of the age of the board. Options are fewer.
http://www.asus.com/Motherboards/P5N32E_SLI/specifications/
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007671%20600005560&IsNodeId=1&name=Core%202%20Duo