Re: 120mm Fans
120mm fans are far superior in my opinion. They're quieter (spin at a lower RPM) and move more air. I have two 120mm fans on a fan controller turned pretty close to all the way down, and I can't hear them at all. To give you an idea of how my computer sounds, I can't hear it over the air conditioner in our house. And that's not loud. It is very, very quiet.
Of course, with them turned that low, my temperatures aren't the best... My CPU (as of the summer in Alabama) likes to idle around 36-38 Celsius, motherboard at 37-39 Celsius, and my Seagate HDD at about 34 Celsius. Those are fine, though, and load temperatures aren't that bad either (nothing over 50 Celsius, usually, sometimes the CPU might hit 51).
All in all, I recommend one 120mm over 2 80mm fans if you are concerned about noise.