Ducky,
So you're saying it worked at 1.8? If so that's great. (Since you're able to run at 1.8, I am assuming you're running an FC-PGA CPU with voltage-jumpers. The Slot 1 CPUs would not be set at that high a voltage. Same applies to the non-voltage jumpered slockets - with one of those the CPU automatically sets a lower voltage than 1.8.)
If jumpering to 1.7 V doesn't work, then just stick with 1.8, and buy a good fan and heatsink like a Golden Orb or something better. Look harder for the revision number. The lettering is quite small and easy to miss. Sometimes it's only a sticker though I'm told.
Out of interest's sake, which slocket are you using? Actually, you still haven't said which PIII 500 it is. Hard to give real advice if we don't know either.
cisco,
A Celeron 400 should run fine on just about any P2B. It's the Coppermine's that are the problem.