I have tried and failed to purchase a PCI Express wireless card that has 802.11n capabilities for my Dual Core G5. So instead, I purchased a D-Link USB DWA-140 wireless dongle, knowing that all their chipsets so far have worked with the AirPort Extreme. Just plug it in and download the driver from the chipset manufacturer Ralink. (I used RT28xx:188.8.131.52b -- 5MB download.)
Once installed, you will need to restart. Configure the new network location (in the Network System Preferences panel), and then find the app called USBWirelessUtility that has been installed. It's as easy as that!
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