Better hold it right there! A 300W power supply is the mininum
requirement and you better test it before doing anything else!!!
I don't know if you will fall a victim of the 133 mhz FSB crash
syndrome
(cannot run your Athlon XP at normal clock frequency, required
to underclock because only stable with 100 mhz FSB).
If you do fall victim of the 133 mhz FSB crash syndrome, then it could be caused by an outdated BIOS.
Don't completely get rid of your Pentium II 400 mhz just
yet!!!