> What would happen if I, say, put a 100MHz clock to the PIC?

It may work, but unlikely. I've accidentally PLLed a 19.6608
MHz crystal on an 18F4520 and it did run at 78.6432MHz. Some
basic tests seemed to show it was working, but I didn't try all the
modules. It didn't appear to be damaged and worked perfectly
with the correct (9.8304MHz) crystal installed. You may get away
with some slight overclocking, say to 20.48MHz and still stay in
spec. A -20 PIC is guaranteed to run at 20MHz and that's what
Microchip guarantee the specs at. It may fail at 20.2MHz, it may
work at 30MHz. I don't actually know what their factory tolerances
or expectations are. For a one-off you could take a chance. Not
something you should do for production though

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