Printer connected to Mac only works if other Mac is turned on
Probably the printer is connected to your iMac G5 and you are sharing it over the network. If you use this shared printer as your standard printer at the Gen. 6 iMac, it will quit working as soon as you turn off the iMac G5. Connect the printer to you WLAN and configure a new printer at the Gen. 6 iMac.