Hey guys, I see we have some computerphiles here too :)
I have tried all the fans listed above including the Enermax Whisper, which is falsely advertised as 'Whisper' when in reality it is much louder than standard Power Supplies. The Thermaltake Volcano 6 that I had previously had a cheap fan which also made lots of noise. I splurged on the silent power supply, silent case fans, Flower cooler for CPU and hard drive enclosure for my new Maxtor ata133. I have to put my ear next to the unit to hear if it's on... About 26db's, well below the measurement for a quiet room which is 28db. I'm running XP1700, 256 DDR, Geforce2 Ti200 32mb and 40gb maxtor. My Samsung 900NF makes more noise that the case.. I'm bringing the monitor in this week to see if it can be fixed (high pitched humming noise.) although this might be normal with the aperture grilles..
Yeah, my 3 year search for a quiet clone PC is now complete!
RE Mac G4... They're already damn quiet, but you could add the hard drive enclosure to further silence it.