I want to do this to increase the speed of my sawtooth. I think that the dual 500mhz was the last apple processor that is compatible with the sawtooth because they both run on 100mhz busses, correct?
With a little tweaking (soldering will be needed), you can fit there CPU daughtercard from DA (less tweaks needed) or Quicksilver (more tweaks needed). It's also possible to overclock system bus to 120 MHz.
Third way is to find for cheap third party CPU upgrade. Some of them, like Sonnets are auto adjusting to system bus speed - would work with Sawtooth with 100 MHz bus or DA, QS with 133 MHz bus.