The only Airport card that is compatible with that system is the original Airport card, not the Airport Extreme card (I think there's a physical incompatibility which prevents using the AE card in a Mac that only takes the original card). The people in the Apple store were correct. Unfortunately, you can only get the original Airport card used or on eBay. However, you can purchase a PCI card from Buffalo Technologies which uses the Broadcom chipset (the same used in the Airport Extreme card) and it will be detected by the latest Airport software.
Here's information on the card:
This would be much cheaper and easier to acquire than an Airport card in my opinion.