The middle spot depends on your case and components. Different case designs need different levels of airflow to cool the same components. Hotter components (faster processors, more disks etc) need more airflow than cooler components (eg laptop processors and SSDs).
Significantly, the nominal dB and airflow ratings of fans often have little relation to how they behave once in a case. Generally if you're looking at fans in the 20-30 dB range then they'll be quiet enough for most people in an "office" type environment. You'll want quieter fans if the computer is going to sit in a living space. Bigger fans will also push more air (higher CFM) for the same level of noise than a smaller fan.
There is, in short, no "once size fits all" answer to what is actually a very complex question.
I would strongly recommend you read Silent PC Review, where they review many components and you'll get a better feel for what makes a good (and bad) fan (or other item).
1In addition to "loudness" of the fan itself, fan's place in computer chassis is also important. Placement of air holes, vibration and turbulence can make significant impact too. – AndrejaKo – 2010-09-26T11:01:09.833
It's just 2 120mm fans, one at the back and one at the front. – Jonathan. – 2010-09-26T11:01:58.983