As a matter of curiosity, how fast can you overclock a PIC with
injecting an external clock signal?

  I'm experimenting with PIC16F877-04/P, and been injecting into CLKIN
from an EXO 16MHz clock generator.  Usually use the 4 MHz divided output
from this but sometimes I select the 16 MHz output, when the PIC still
seems to work perfectly well.  Doesn't get warm or anything.

  I know the spec sheet says the operating speed is dc to 20MHz, but am
noting the label on the chip itself -04.

  These are single chip computers made via modern fabrication methods.
Thus small parasitic capacitance and miniscule lead lengths should allow
very fast operation.

 Peter