The folks at Engadget have posted an exhaustively thorough article
on an upgrade they performed on a 400mhz Powermac G4. Specifically, they replaced the stock processor with a PowerLogix PowerForce47 G4/2.0GHz, with 512K 1:1 L2 Cache per processor. In the process, they came up with more comprehensive set of instructions than the rather anemic list that shipped with the PowerLogix processor. After a short trip into the wonderful world of firmware, they've got their Sawtooth purring along happily with its new 2.0GHz heart intact. Check it out.