It seems that what we thought was impossible is actually possible. I am typing this message from a virtually silent 1.4 dual MDD G4. I've achieved this with the following modifications. First was the power supply fan mod, as shown here starting at post #11. Second was replacing the main CPU fan with the same fan used in the PS mod, the Antec 3 speed. Both fans are being run at the mid-speed switch selection and so far my CPU temperatures have been very acceptable, from 33C-38C at idle and while typing this message up to 50C-55C while under any load I've managed to expose it to. Finally download Chud Tools and enable NAP mode. This system is as quiet as my 2012 mac mini and no noisier then my G4 cube, which also has an 80mm version of the same fan. Over all the mod has achieved a very pleasant and a joy to use MDD.