Is there an airport card that I can install to upgrade to "N"
Does Apple make an updated airport card that I can replace the existing G card with to utilize my N network?
there are some vendors that sell 802.11n transceivers that fit in the CardBus slot, they seem to advertise PowerBook G4 compatibility (I have no personal experience using one)
although it doesn't physically replace your existing AirPort card, it's still somewhat internal in the slot - however usually the end of these cards usually sticks out a little for the antenna
There is a PCMCIA (PC Cardbus) that is 802.11n compliant. Yes, it fits in the cardbus slot on PowerBook G4's and it's available at OWC. You have to make sure that you either disable the internal wireless card or take it out. You can't run both at the same time.
To the best of my knowledge, there is no internal 802.11n card that will work with a Powerbook G4. there are, however, 802.11n usb adaptors that can plug into these computers' ports. I haven't used these myself, so I can't vouch for them, but here's one I've been looking at online for a while: