I would suggest looking into a Belkin, D-Link, or Linksys (Cisco) USB to Ethernet Adapter. They are much less of a hastle than trying to make sure a PCI or PCI-X card is needed in a G5, and usually cheaper.
I think you misunderstand TEG's suggestion. A USB to Ethernet adapter gives your Mac a new Ethernet port, so you don't have to use the malfunctioning one.