Wow, some "vintage" overclocking!
Hehe, I ran all my PII 350 between 450 and 500Mhz. So you
are right in there.
There is a way to unlock the multiplier on those procs by the
way. I wrote an article or two about it years ago. If you have
a good motherboard that lets you adjust the Multiplier, you might
be able to do some more out of it!