My situation is very similar to described earlier, but with some diferrences.
Computer G5, SN: XB3480R4NVB, running 10.5.6, A 1038 display, 2 GB RAM, two drives.
Just froze on me a few days ago, After force quitting with power button, when trying to boot chime sounds, fans work, no display, light steady. NO power to keyboard or mouse, CD/DVD not powered either, so I can't boot from disk.
The only change lately was that I connected Beringer audio- digital converter to the FireWire port more than a month ago and computer was working fine after that - until now.
I tried to take out one pair of memory, then another - no effect. Tried the PRAM battery - it is pretty fresh. I can't zap the PRAM from keyboard - keyboard doesn't work and I have an extra Mac keyboard, changing it doesn't help. Also pressed PMU/Cuda button (as recommended on the net) - didn't change anything.
I didn't try to reset the video card yet. The fact that neither the mouse, nor keyboard (nor CD/DVD) work, I suspect it is Power Supply. I would really appreciate getting some suggestions!