This is a blog dedicated to quieting the MDD. I have several posts there as Jeff B.
http://coolstuff.wordpress.com/2007/02/19/powermac-g4-mdd-cooling-and-noise-reduction/

Its the best pool of info I have seen. Used all SilenX in mine and plugged the 120mm replacement into the mobo rather than standard 4-pin psu power. That way the fan only gets loud under heavy load but even then its still much more quiet than it is on a 4-pin. On the 4-pin it will always run at full speed. I did plug the 60mm psu fans into the 4-pin because the new fans only blew 18 cfm vs. 35+ of the stock fans. I also added a 3rd 60mm to the back exhaust of the psu to help suck out more heat. Never had a heat issue. PSU always feels more cool than warm and is very quiet. 16dba.. anything under 20 is inaudible if your tower is on the floor.