It can be lots of things - I recently relocated my Powerbook to a new desk, that is in a different position than where it was before. This cause the signal to drop from full to only 1-2 bars, and I keep having problems with disconnections to the internet. If I move a couple of meters back, to where the old desk use to be (a space now occupied with my bed), I get a full signal again.
My Windows laptop, sitting right next to the Powerbook, gets a very decent signal (not full, but at least 3/4).
These are the strange things about wireless networks. I would suggest moving the G5 around a little bit
Me? I'm going to buy a range extender for my router to ensure I get full signal everywhere in the house from now on.