I'm surprised you can run a Celeron 1.2GHz processor on the P3V133, since your motherboard does not support Tualatin processors (and it doesn't support Coppermine processors properly either for that matter...).
Since your Celeron should be clock locked, it should actually be running at its full frequency; the BIOS is simply displaying an incorrect speed. Check it with WCPUID to make sure.
In any case, you should upgrade to the latest BIOS,
Version 1003 Beta 002.
Don't let the beta status put you off. Any BIOS upgrades made by ASUS after a board has been discontinued is almost always left as beta, even though they are perfectly stable.