I have a heavily upgraded dual 1GHz Quicksilver - and have spent many a Saturday trying out different mods and fans to see what works. First things first, the replacement PSU's that you mention were only available to the Mirror Door G4's. That said - there is much you can do to reduce the fan noise and cool your tower at the same time.
First of all - do your research.
Do a google search for "quieting a Quicksilver" (and similar words).
You will find (literally) hundreds of entries.
I've provided you (below) with *some* of the links you should look at to reduce excess noise in a G4 Quicksilver. They all provide interesting and useful ways to achieve this - as well as some unique Mods. Most contain photos so they're very easy to follow and understand. Have a flick through them to gain a basic understanding of what can be done. Then follow these:
(From experience I would recommend to do the following)
1. Cut out the fan grille on the power supply. (keeping the plastic outer grille).
2. Use the trick of putting pieces of foam between the fans and screws - to reduce fan vibration noise.
3. Consider buying new fans (Especially the big 120mm fan that blows onto the PCI cards)
I used an aluminium Evercool fan - connected to a fan controller and operating at half speed. From experience the 60mm CPU Sunon fan (next to processor) is very hard to find a better replacement for - and unless the bearings are shot I would probably leave it. (However I replaced the 60mm CPU fan with a Sunon KD1206PTB1. This is probably the only fan to get in order to replace the CPU fan.) Bear in mind that the fan inside the power supply is thermal controlled (kicks in when the temp sensor gets to a certain temp.) Probably best to leave that one as it's very tricky to get out - and dangerous too! Again, unless it is very noisy or its bearings are shot.
4. And lastly, really consider reversing the big 120mm fan so it sucks air into the case - rather than what it is doing now which is trying to push air out very unsuccessfully (and non-quietly) through the holes and out the back. For me, this 4th step provided the most effective noise reduction. For some stupid reason, Apple didn't design it too well. And everyone has complained about how it doesn't really push air too well out the holes of the side of the case. I (and others) have found that if you reverse it (so it's pushing air onto the PCI cards) it effectively sucks cool air up from below the case quietly. But remember that if you do this - you will need to also reverse the CPU fan so it is blowing the other way - sucking air across the heaksink and blowing it out the back.
With the addition of replacing all the fans with quieter ones and connecting them to a variable fan controller - I performed all these mods and now have a much quieter (and cooler) Quicksilver.
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> Various articles on quieting a Quicksilver at
www.xlr8yourmac.com: (6 links)
http://www.xlr8yourmac.com/systems/quicksilver_noise/quieting_quicksilver_noise.html
http://www.xlr8yourmac.com/systems/quicksilver_noise/quieting_quicksilver_2.html
http://www.xlr8yourmac.com/systems/quicksilver_noise_2/quieting_quicksilver_3.html
http://www.xlr8yourmac.com/systems/low_noise_fans.html
http://www.xlr8yourmac.com/G4ZONE/G4_fan_noise.html
http://forums.macnn.com/showthread.php?t=209451
