WEP is Wired Equivalent Privacy (or something close, definitions seem to vary, try googling define:WEP and see what i mean). Either way its an encryption system for wireless networks, so a WEP key is an encryption key for WEP protected network.
Grimm, your WEP key should be listed in the device where you turned on the WEP encryption, probably your router. Even if you've entered it on your G5 to access the network, I've never found it listed, you just enter it once to access the network then the system remembers it.
It should show on the router/airport/wireless access point on your network. Just log into it and check the section for the WEP key. You won't see it on the Macintosh because it's supposed to be hidden, like all passwords.
Do you mean to create an ad-hoc wireless network? I don't think you can use the AirPort card in the G5 as an access point but I may be wrong. I think you will need an Airport base station or other access point. If your G5 is plugged directly into your cable/dsl modem then you are missing some hardware.
Just open System Preferences, and click on Network. Then select "Airport", to show the Airport settings.
If you're using Tiger, you then select your network in the list, and click "Edit" (a small button below the list). In the dialogue box, you check "Show Password". You may have to authenticate to see it, but probably not.
On my 'puter, it shows the Hex equivalent password, which is probably what you want anyway to get your PSP to connect.
I don't remember the exact way to do this in earlier versions of OS X, but I do know that it can be done, and that the information is in the Network panel somewhere. I used to work in a place where we often had visitors from other branches, and I always managed to repress the memory of that particular password, so I've looked it up lots of times...
If you don't find it, start the admin application for your access point (Airport Administrator in /Program/Utilites if you're using Airport), and log in to your router and look it up there.
You could also try using "Keychain Access", as the password is stored there too. In Tiger it is on the keychain "System". You'll have to authenticate to see it in plain text though.