Is your BIOS updated? The e8400 wasn't even around when your motherboard was originally built, if you haven't updated, your motherboard won't properly support the CPU.
Go here http://support.asus.com/download/download.aspx?SLanguage=en-us
and fill in the blanks, select your OS, and then look over the BIOS updates. There's one for that mobo dated 4/28/2009, so odds are you don't have the most up-to-date version on your system.