I'm judging by your first and last posts, but just to check:

You have plugged in the little plastic T shaped aerial that comes with the PowerMac G5, right? It plugs into the back of the machine.

You've also plugged in the end of the wire inside the case, into the airport extreme card? And you're sure you plugged in the airport wire, not the bluetooth one?

The "do I need to plug in an aerial?" FAQ entry is referring to iBooks, not powermacs; you most definetely DO have to plug in the aerial and its wire, on these machines.

Your last post; it sounds like you don't have a connection from the airport extreme card, inside the case, to the aerial on the outside of the case. In which circumstance the case itself will block your signal, as you described.