The most likely culprit is the hard drive if it posts and won't go any further and disk repair won't work. If you can boot from the optical drive, then you should be able to take a look at the hard disk and see if it is working properly. If it is, you cann just refortmat and reinstall. If the HDD is not working, all is not lost since the first gen G5 is pretty easy to get into and swap out the hard drive (unlike the Frontrow-enabled G5 and Intel iMacs). If you don't have Applecare, a hard drive swap might be worth trying.