It depends on which version of 600 MHz iBook G3 12" you have. If you check the bottom of the machine, the model number is in the small print near the hinges. If you have the A1005 version, you can use the 900 MHz board in your machine. If you have the M6497 version, it's not directly compatible, and you would have to change some other parts.
What's the verdict on the model number?