Your hard drive may be getting ready to go belly up.
I have a Powerbook G4 that did that last year; spent more and more time proudly displaying the spinning Beach Ball of Boredom, and then started having boot problems. Then the hard drive finally croaked for good, but only right after I was able to copy it - I'm a lucky boy...
I had had Apple Care on it; although it had expired more than a year previously (the machine was more than four years old), they replaced the drive for free.
Runs like a champ, now, but since the old non-Intel chip won't run Snow Leopard, I'm currently waiting for my replacement 27" i7 iMac...
So will the G5 work with another HDD (compatible interface) I get from the store or do I have to pay an extra $500 to get one from an Apple store?
I second the hard drive trying to die. That is classic symptoms, regardless of OS, though Apple’s OS seems to go on a bit longer before it collapses under the dead drive.
If the original poster gets DiscWarrior, it will tell him if this is the case.
But I strongly suspect his HD is about to blow.