I too have been watching the IIsi speedup reports and plan to upgrade in
the next few weeks.  The plan I have is to build a small board with a few
different crystals on it and to be able to switch between the different
speeds using a front pannel switch.  This way I can get the speed when
I want but I can also run at slower (stock) speeds it I experience any
compatability problems with any applications.  I don't expect to be able
to switch clock speeds with the system running, but if I can switch
without any lock-up problems, then I could switch to 33MHz when needed
and put it back to idle (20 MHz) when not needeed.  This would further
reduce the wear-and-tear on the CPU even with a heat sink.  Of course
I would not want to run the different clock signals through the switch
but use a chip or two on the board to select the frequency desired and
route it directly to the mother board.

I haven't started probing around inside my si yet.  Does anyone know the
voltage level to power the crystal oscilators?  