that's not exactly how it works. Every 10 decibles is what we perceive to be double the volume, hence 40db is twice as loud as 30db, or four times as lound as 20db, or eight times as loud as 10db. 1db is I BELIEVE the smallest change in volume that may be percieved by humans.
You can not add the db ratings of your components together in order to get the sum db of the entire system. for instance, say you have 2 120mm panaflo L1A fans that run at 30db each at full rpm. You don't have 60db which would be quite loud, you actually have around 32 or 33 db (just my best guesstimate). The trick to silencing any computer is locating the loudest single component and removing it or finding a way to dampen the volume.
I do reccommend you visit silentpcreview.com--read the articles and the recommended list, then visit the watercooling forum. The people are mature and helpful (moreso than most other forums I visit).
As for watercooling for silence, I suggest you use an eheim 1048 pump and only cool the video and the cpu. Put a passive Zalman HS on the nb. You can suspend the hard drives (samsung spinpoint 160s seem to be the silent choice atm) to further eliminate noise. You also want a quiet PSU--the Noisetaker 470 from Enermax is a popular choice for people who want quiet but still want higher wattage performance. You should then turn off as many fans as possible and undervolt the others until your HDD temps get too high and then back off slightly. Those papst fans you have probably don't need to be undervolted though considering how quiet they are already and how little cfm they push.
Bruce