What have you done so far?
Did you boot up from your install dvd and format the disk with disk utility? What OS did you install? Where did you get the dvd?
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Seems like you get some bad instructions loaded from the hd or you have a bad memory card.
I'm not all the familiar with diagnosing the G5.
1) Try reseting the smu. What you do depends on you model of imac g5.
http://support.apple.com/kb/ht1767
2) Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
reset-nvram
set-defaults
reset-all
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."
http://support.apple.com/kb/TS1812?viewlocale=en_US
How to eject a cd from the internal cd drive:
eject cd
List of devices:
devalias
List of variables:
printenv
3) It's seems to me you got a memory error. I'd take out a memory stick & try again. Use the option boot.