Over at ethernetworks.de there’s a really good description of how to overclock an Apple iMac G4. Unfortunately, their table of resistor positions only holds true for the 167MHz USB 2.0 model iMac and mine is slightly older and has a 133MHz bus.
You can work out the appropriate PLL resistor settings by referring to the datasheet for the MPC7455 (see Table 17: MPC7455 Microprocessor PLL Configuration Example for 1.0 GHz Parts). However, figuring the combinations out can be a bit dizzying, so after working out the settings for 1.27GHz, I figured out the rest just in case I needed them in the future.
‘R’ = resistor present, ‘-’ = no connection.
Please only use this information after double-checking that I’ve got it right. The iMac G4 is probably my favourite design of computer and I don’t want to be hearing that you’ve stuffed one up using the information I’m providing. If you’re uncertain about anything, just don’t do it (although, I must admit that a jump from 1GHz to 1.27GHz does make a noticeable improvement to the machine).