Sounds like a problem with the antenna. The fact that you can get a signal at all indicates that the card and associated software is fine. Having someone else try validates your base station. Swapping the cards was a good idea, also.
In other words, I'm afraid you're boned. I feel your pain, though - I had to send my iBook in only a few weeks after I got it when the lid-latch broke. The bad news is that to replace the antenna requires a pretty thorough disassembly of the iBook; the antenna is in the lid, behind the display.