Which keyboard should I buy for my Mac PowerBook G4?

any keyboard which uses a USB connection will work with your mac. i advise going to a large computer retailer and trying out a bunch of keyboards until you find the one you like.

Do you mean an external keyboard for when your laptop is stationary, or a replacement internal 'attached' laptop keyboard?

An external keyboard for when my laptop is stationary; to use with my bigger monitor.

Well as Sam said, I'd suggest you just go 'feel some up' (d-:) to get a feel for one that just... fits you. It depends if you want something 'heavy' and 'clicky'; something more like an old fashioned typewriter feel, something light and 'invisible' to your fingers, something thin and slim - there's a lot of different types, and what matters most is really how it feels to you. I myself use a Logitech DiNovo:
http://www.logitech.com/index.cfm/keyboards/keyboard/devices/192&cl;=us,en

Another I really like using at the various places I have used it is the Kensington Slimtype:
http://us.kensington.com/html/5463.html

Again, I wouldn't order one online until you've physically tried it out - it's ALL about how it feels to you in the end.

I would look into getting Apple's bluetooth keyboard (that is if your MacBook has bluetooth) -- http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore?productLearnMore=M9270LL/A

As an addendum to my earlier answer, ttoone is right too - I own and love several Mac keyboard. I just happen to know that in any given conversation, I seem to be the only one who likes using the Apple-made keyboards, everybody I know seems to find them uncomfortable - hence why I never recommend them off the bat.

The correct answer is: http://www.amazon.com/Microsoft-Natural-Ergo-Keyboard-4000/dp/B000A6PPOK

This is your winner, the software is good, and the keyboard is mgnificently comfortable. There are days I think microsoft should give up writing software, at which they are mediocre, and focus on making input devices, at which they are gods striding the earth.