??
Adding RAM to your Powerbook is a good thing.
Some of the new machines seem to be having a problem with the OS and networking locking up when there is more tha 1 Gb installed. Presumably this is a software problem that Apple has/will fix.
THe Powerbook does not require matched RAM, you can have 2 different capacity modules in it.
Depending which 1.67 GHz G4 you have you may need DDR PC3200 or DDR-2 533 Mhz SODIMMs. I recommend you go to a reputable dealer who tests and guarantees compatibility with your model Mac, offers a lifetime warranty and a no-cost return if it doesn't work.
Thanks
Trevor
CanadaRAM.com