If you are giving this computer away or selling it, it is your obligation to give the purchaser or recipient the original Install Disk and any other disks that came with the computer. You can get replacements at nominal cost from Apple if you call and give your serial #. If you don't, you are selling a crippled computer. (Of course, this makes the entire procedure below moot, since you will have the Install Disk which you can use to do this.)
There are no keyboard combinations for this and you won't be able to use the disks from your other computers if they weren't the full retail versions usable on any machine. If you have a clone of your G5 on an external drive, you could boot from it, or if you have another Mac with the same OS (Tiger or Leopard will probably work as well; I don't know about Snow Leopard), you can boot the two computers in FireWire Target Disk Mode. This will effectively turn the G5 into an external drive.
http://support.apple.com/kb/HT1661
Open up Disk Utility on the Mac you are booting from, select the G5 Volume and go to Erase>Security Options. Choose Zero out Data 1 pass. This will wipe the Drive completely and securely, but it will not reproduce the state it was in when it was completely new, which would have opened Setup Assistant. At this point, the computer will not have an OS installed.