Most of these G5 iMacs did not come with the card installed. It's a five minute job to do it. Here's how: [guide|874]
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Your machine uses a standard AirPort Extreme Card. It can be found on eBay or here: http://www.amazon.com/Apple-M8881LL-AirPort-Extreme-Card/dp/B0000899ZB/ref=sr_1_7?ie=UTF8&qid=1296788429&sr=8-7