Apple's original AirPort cards can be as scarce as hen's teeth these days.

Well, she's going to need an AirPort or AirPort Extreme card - but to give you more-specific advice, we’d need to know how old that "older iMac" is. The first iMac that could take an AirPort card (aka 802.11b) was the iMac DV, introduced in October of '99; the first iMac that could take an AirPort Extreme card (aka 802.11g) was the 17-inch LCD-panel-on-a-chrome-stick-on-top-of-a-giant-Hostess-Snowball iMac released in February of '03.

Unfortunately, the older AirPort cards are hard to come by these days - your best bet is eBay. The AirPort Extreme card is still available from Apple, however, for $49. Whichever you choose, unless you're the handy type you should probably have a professional install the card, since it will require taking the iMac apart.

On the other hand, if you don't mind not going the internal all-Apple route, and you also don't mind dedicating a USB port to Wi-Fi, there are a number of Wi-Fi adapters that'll plug into an iMac's USB port, such as the MacWireless 11G. But before you buy, make sure to ask about Mac OS X compatibility - most USB adapters should work just fine, but better safe than sorry.

Finally, if you're interested in extending your daughter's Wi-Fi range and don't mind going Deep Geek, here's a site you just gotta check out.