what do you use the pc for, gaming, browsing . . . .
and to be frank i dont really recommend upgrading cz your mobo is too old, and your gpu is old and weak too, i recommend you to save your money and do a full system upgrade later
anyway here is a good value cpu if you insist:http://www.newegg.com/Product/Product.aspx?Item=N82E16819115057
but again, i dont recommend upgrading, save up for a whole system upgrade better for you