It is a little difficult to give you a specific answer since you don't say whether you have Airport card or a third party wireless PCI card installed on this PB.
If you have an Airport card, the Airport Setup in the Utilities folder should be able to help.
If you have a third party wirelss card, you may need to install the accompanying software and then presumably it will put a preference in the System Preferences window. Click on that and follow any written instructions you may have with the card.
I have a similar problem i was hoping you could help me with... Apple has declared my 2001 g4 "obselete" and won't help me. I recently bought a third party wireless card (ARIA) and when installed it recoginzes my cisco/linksys router, but cannot connect using the password i have, even when i use a $ in front. The linksys folks (much more helpful than the mac folks) determined that it may be that the linksys is running at 64 bit, and the g4 only has 40 or 128 bit for the WPA encryption. We tried configuring the linksys with no security, and could still not log on... please help! my computer is not obselete!