best thing to make sure you have a quiet case is to get a good aluminum case
Aluminum cases are the noisiest (save for those unecessarily expensive fully-shocked desk-top cases). Go with good, heavy steel, and use some vibration gaskets.
As for fans, the size matters, but also the make. Buy good quality fans and pay attention to the noise dBs. My case for my main home machine as 4 80mm Stealth fans that run at less than 20db, which replaced the stock 80mm fans that came with the case which were running at 43 dBs.