what bios is the mobo on? if it's not rev 1005 or later, got a spare P4C? put it in and flash the bios.
http://www.asus.com/support/cpusupport/cbios_dl/cbios_dl.aspx?MB=P4S800-MX&CPU=P4-3E%20GHz%20(800%20FSB,%20L2%20cache:1MB,%20HT,%209 0nm)&BIOS=1005