Overclocking Older Systems

This is the sort of thing that separates the gods from the atheists. It's great.

Try this: Get some big, fat heat sinks and epoxy them to everything that gets warm. Get a blower with a hose and cut some more holes in the case for it to go through. If you can cool the air somehow, even if only by blowing it over ice or (better yet) dry ice, so much the better.

You'll probably get obvious soft errors before you fry the chip anyway.

but I'm running a 600 chip currently stable at 900.
you can possibly get to 133 or higher, but every chip is different.
Cooling is everything.
voltage is second.
Cool as much as possible and then raise speed until you get software errors then try raising the voltage a bit and keep going until you get unacceptable errors. Take it in small increments and then back off. Get a temp sensor and watch heat build up and it's relationship to errors. Keep good notes. 486 processors are pretty much free, so there is not much to lose. There are great articles everywhere on clocking. Just use your favorite search engine.