I did a quick google search on the 8400 processor. The good news is that it is a good overclocking processor. However, to get the most out of it under air cooling, you'll need something better than the stock cooler these are the recommended cooler and backplate:
http://www.newegg.com/Product/Produc...82E16835233003
http://www.newegg.com/Product/Produc...82E16835233019
Anyway, hope this helps.