If you want the best CPU are C2D E4600, E6600. You maybe can't find them because they are old. Or go to ebay.
Looking at the ECS website, it says this board can go up to 1333 FSB, meaning you could drop an e8400 into it. But, as the other posters have pointed out, unless you can get one cheap (and second hand...risky) there is no point in putting money into your rig.