It is possible to overclock my 400 Mhz Celeron; I have an Acorp maiboard and the only possible FSB`s are 66 and 100 Mhz(in fact it is only one jumper wich does the job) .
Celeron A/400/66 works ok at 450/75, max 500/83
Default voltage 2.0V, max 2.2V
Mine works best at 450 using an intel cooler from an old celeron
If you can't do it from bios try softFSB, works for me (acorp MB too)