You will be pretty safe if you continue to increase the FSB setting until the processor locks up, then back down to the next lower setting. Run for a week or so, then try to decrease the voltage setting until the processor locks up then increase to the nest setting. Run another week or so and try to increase the FSB setting again. You may get lucky and be able to jump up to the next setting and maintain stability.
The PII/PIII's have a built in thermal overload, so as long as you keep the temp under 70°C, it will last a long time. DO NOT go over 2.3 volts for the core voltage setting, or you will decrease the life expectancy of the processor.
I keep my PII-450 running 558 MHz @ 2.2 volts while under constant 100% CPU load at or below 54°C or it will lock up.
According to a program called SiSoft Sandra that I got off a cover disk, the only supported voltage on the processor slot is 3.3V. The motherboard is from Quantex (came with the computer) Does this mean I shouldn't overclock?