The new 0507 BIOS for the P5B Deluxe, dated 8/10/2006, has two new and exciting features:
Provide better maximum overclocking.
Add the ability to adjust the multiplier of most Conroe CPUs even if they are not Extreme Edition.
Check which bios you have you maybe in need of it.
edit: nvm you have the SE. I don't think your board is meant to oc.