same problem. iMac G5 250GB 2GB RAM Tiger. Got stuck on grey screen starting up, fan spinning at top speed. Forced shut down/restart. Starts up, chime and then google notifier sounds there. Time machine backed up. So system running. But I have a black screen now. Tried hooking up external screen, and resetting PRAM. No success. Any ideas? I'll check capacitors if you think this is a possible problem.
Cheers,
Darko