This is sort of an answer to your question.
I wouldn't get a PCMCIA card, rather I would look for the card from Focus that combines ethernet & graphics card. It goes inside the PowerBook, which is a real pain to open up.
As a partial answer to your question, choose an ethernet card which accepts the RJ-45 directly, not one which has a dongle connected by a very weak connector. I had one which worked on my 5300 back in 1996. I'll post a picture of it if I can find it at home.