I agree with the others. Bad idea!!!! If it was a PowerMac, I'd say go for it, but certainly not on a laptop.
Basically what you have to do is, fine the G4 Model (7455,7400, etc...), then you'll have to search around and find which configurations the transistors go on the logicboard. If put them in certain ways on the logicboard, the CPU will run faster or slower depending on how you have them. So you'll have to unsolder some of the transistors on the logicboard, and then resolder them back in the correct way to make it clock faster than 667MHz. Its not an easy thing to do and is not recommended at all for a laptop.
What you're better off doing is, look around for something like a 1 GHz PowerBook (Titanium, or Aluminum) and that will have you set for a while on most tasks.