Which PB do you have?
There is a company that makes antenna kits for the powerbooks and still uses the Airport card,they look a little strange but would have to be more convenient than using an external adapter I would think.
It is the same company that made the swing away handle for the TiBook but the name escapes me right now. I did call them at one point and they were very friendly and helpful and suggested an alternative for me that I ended up using for that job.
Better servvice than can be expected.
Found it:
www.quickertech.com
I have also used USB adapters and they had connectors for external directional antenna.