Do you have an iMac G5, or a PowerMac G5? Connecting the internal Airport antenna cable (as well as the external T-shaped antenna on a PowerMac G5) is critically important - without it, your Mac will get no wireless network reception.
Installation instructions for the iMac G5 20" (original version):
http://manuals.info.apple.com/en/imacG520inchAirPort.pdf
...for the iMac G5 17"
http://manuals.info.apple.com/en/imacG517inchAirPort.pdf
...for the PowerMac G5 (not the recent dual-core models):
http://manuals.info.apple.com/en/AirPort_Card.pdf