1. The 12" PowerBook does not have an AirPort slot, it has an AirPort Extreme slot, which is mini-PCI, not PC Card-like.
2. Lots of manufacturers make big CompactFlash cards, but they cost as much as the PowerBook. I think the biggest at the moment is 6GB.
3. Non-AirPort cards do not work in an AirPort slot, even if they do fit -- I've tried. I do not know if any non-AirPort Extreme cards work in an AirPort Extreme slot.
Basically, your only choice is to use some form of external storage.
tooki