idkew's information is false, actually.
Your computer runs at 667 mhz. That is a 133 mhz front side bus with a multiplier of 5. (do the math: 133.33333 x 5). Your processor is hardwired to run at this speed. Regardless of what speed memory you put into the powerbook, it will still run at 133 x 5. If you put PC166 memory which is specicifed to run at 166 mhz, because your bus is at 133 mhz, it will run at 133 mhz.
the PCXXX number is just a specification. Memory manufacturers make memory and they test it at various speeds. If the memory runs at 133 mhz, they slap a PC133 label on it. If the memory runs at 166 mhz, they slap a PC166 label on it. However, often their settings are very conservative and memory can run faster than it is specified for. If it does not work, your computer will crash. That's it. Your computer WILL NOT slow down, however. That is completely false.