I don't quite see how this would get me the ambient sound lvl in my house. Is it possible to then take the # of volts for the given fan and look up how loud it is? Does silent mean the fan is at or below ambient dba for the room?
Exactly, you will use the fan as a gauge, because you can find in SPCR's fan reviews articles a chart of noise in dB vs. voltage. I'm not sure though how you can relate the dB of the fan and the fact that it is more or less inaudible with the ambiant noise in dB in your house. Maybe the comments in the articles will be more useful, saying for exemple that "at 7v the fan becomes inaudible at 1 meter with a 20dB ambient sound level in SPCR's lab.
I would say +/- 1 is a reasonable accuracy with a proper measuring device, but with this fan gauge technic maybe you can expect to be within +/-2 dB of the real noise.
But in the end, I think like Aris that it will be of little use to you. Maybe it will avoid the step by step process of tackling each time the noisier component, until you find out that your first quiet component bought is not quiet enough now that your computer is close to silent, but apart from deprieving you of that fun journey ...