Well, I knew sooner or later I was going to do this, so I come a little prepared. I was given a 733mhz G4 Quicksilver with a bad power supply unit (PSU), so I'm going to try and install a PC ATX PSU instead, despite the fact that I see I could buy a used QS PSU on eBay for $36 including shipping. I have one question to start. Out of my small ATX collection I've chosen the one shown here:
As you can see, like many PSU's--but unlike the PSU's in G4 towers--it's got a second large fan that blows down into the case--in the standard ATX case design, blowing down over the processor heatsink which usually has a fan itself.
Now if I use this PSU and align it along the screw holes and openings on the back of the QS case, I will simply have to cut a hole for the power plug since everything else will align just like the original QS PSU. The original power plug opening will allow access to the ATX PSU switch, which will be nice, and I'll only have to cut the plastic on the back for the ATX power plug socket, not the metal case itself, because for some reason Apple made the hole in the metal case back there large enough to accommodate a plug socket on either the left or the right, even though the OEM PSU has the plug socket on the left and the plastic outer layer of the case only has an opening on the left. (Just how did that little slip to the ATX standards happen in the G4 metal case design--even though the outer plastic layer doesn't acknowledge it at all? Seems like a nefarious sleight-of-hand by some forward-thinking, yet rebellious Apple designer.)
Anyways, if I align the ATX PSU according to the OEM QS PSU fittings--which is what I plan to--the second large fan on the PSU will be facing inward at the static wall of the case with only less than a half-inch gap between the wall and the fan, which sounds ok.
I could, on the other hand, by doing some major modification of the back of the QS case, including drilling new screw holes, turn the PSU so that the fan faces toward the wall that opens instead. If I did this, then that second large fan of the PSU would be blowing directly onto the large QS processor heatsink. It would be practically laying on top of the heatsink. I don't want to do all that modding of the back, but it brings a question to mind: why don't the G4 tower PSU's have large second fans in that position? It's such an obvious thing to design in for direct cooling of the CPU heatsink. Is there a reason why not? On my particular ATX PSU, there is that metal grill over the fan which might touch the heatsink and have the potential danger of causing an electrical charge to the heatsink, I suppose, but PSU's can be designed with second fans without that potential--without a grill which could touch the heatsink. So why don't G4 PSU's have that internal fan?
I assume that that second fan on ATX PSU's is for cooling the case, or am I wrong? Is the fan actually for blowing heat out of the PSU, and therefore you wouldn't want it blowing warm air onto a heatsink?
Any power supply that's correctly wired internally is going to have that second larger fan pulling air into the PSU, not blowing it out. Otherwise, you'd have the two fans fighting each other, and not cooling the PSU, since both were trying to exhaust air.
That said, if you could orient the fan so it were facing the CPU heatsink, you still should see additional cooling benefits.
If the fan grill were to touch the CPU heatsink, I'd worry about it putting pressure on the CPU, but electrically...no.
Looks like you're going to have to cut some plastic on the back of the case no matter what you do.
Keep taking photos!