Not connecting to local wireless network.

Ok, I can't seem to get these network options right. I have an iMac Intel and an iBook G4 both connected to the internet via Airport Extreme wireless router. I basically want to have it so that when I boot up my computer it will only join my local wireless router automatically and not any other networks around me. These are the options I have set under System Preferences - Network:

Under the Airport tab:
By default, join: perferred networks
My Network

Under the Options button:
If no preferred networks are found: Ask.....
Everything else unchecked.

When my iMac and iBook boot up it won't recognize my preferred network for some reaosn and won't connect to anything. Maybe I don't have an option set correctly?

Both are running latest Mac OS 10.4.8