Here are a few things to think about:
When the dB levels increase by 10 dB you hear that as twice as loud. A difference of 3 dB is just audibly louder. If you are into sound systems, it takes double the watts to get a 3 dB increase, and 10 times to get a 10 dB. i.e. you can't hear the difference between a 100 watt amplifer and a 150.
A typical quiet house is in the 40 dB range. Case mfg's talk about 25-30 dB but realistically they are not much better than 40 dB, which means a good PC can be built that is barely audible. In a home 50-60 dB is going to be annoying.
The other tidbit is that two 30 dB sources add to give you 33 dB (or +3 dB, when you have two sources at the same dB). So five fans are going to sound about twice as loud as 1 fan, if they are all the same noise level.
The key to keeping the noise down and the cooling up is to use large fans and slow speeds. My suggestion is to consider the Antec Sonta II case, and try to minimize the # of fans -- ideally only the standard 120mm on slow speed, in addition to the two PS fans -- with typically only one running.
Ron