I have just converted an ATX power supply to work in my G4 MDD Dual 1.2 FW800. I used an APOLLO 300 Watt Micro ATX Power Supply. It's day two and everything is working fine so far including DVD drive and firewire 400. I haven't tested FW800 because I don't have any FW800 devices. I found a post on the internet stating that a 300W power supply should be sufficient without the Apple Display. The internet is full of good information so I went for it. The computer is working, who knows how long it will last? You? You should not do this, you have been warned, there is danger and electricity involved, this list worked for me, you are on your own.
'''Here is what I did:'''
# Remove the old power supply from the G4 MDD
# Cut the wire harness from the old power supply as close to the power supply as possible
# Strip 1/2" each from the end of the 24 wires from the old G4 MDD power supply
# On the plug from the new ATX power supply, one at a time, cut and strip 1/2" from a wire, matching the color with a wire from the old G4 harness
# Twist the matching color wires together, one at a time, solder, and cover with heat shrink. There will be a few black and yellow wires from the MDD that won't have a matching partner on the new harness. Twist two of the same color together from the MDD and use one from the new ATX. You only need to do this 3 or 4 times
# The white wire from the MDD (pin 14) is for an Apple Display and isn't used in my configuration. Cover the end with shrink tube and forget about it
# Check all of your wires for color matching and heat shrink
# There will be a few wires from the new power supply that don't get used, I think there were a couple extra red wires and a grey one, you will figure it out. Shrink wrap the ends and keep with the harness
# After checking every set of wires to make sure that the colors match and they are covered in heat shrink, plug in the power supply by itself and make sure it doesn't fry before checking it in your computer
# After checking the power supply powered up by itself, plug the power supply into the computer and see if it powers up. If it does, you are a hero. If it doesn't, I told you not to do this in the first place.
'''Mounting the power supply'''
I used epoxy to mount four cable tie mounts to the top of the inside of the case, then used cable ties to hang the power supply from the mounts. The PSU fits in just fine and clears the video card nicely. I'm thinking about adding a PCI slot fan for more ventilation but we will see. My G4 MDD has a new lease on life for about $30 USD including the cable ties and mounts.
Also, I picked up a 6" extension Y cable to extend the power going to the hard drive. This proved immensely useful when it came time to stow all the wires behind the DVD drive and up to the power supply.