I would suggest spending a bit more, and replacing the fans with these: http://noctua.at/main.php?show=productview&products_id=57&lng=en
I replaced all the fans in my system with Noctuas over the weekend, and those are the quietest of the lot. (and replacing everything in the case with Noctuas made a significant difference to the noise levels)
At 1200rpm they are rated at 17.8 dBA, which is reasonable for that speed.
When you use the low noise adapter to reduce the speed to 900rpm they are rated for 10.7 dBA, which is extremely quiet. The new Mac Pro which reviews are calling "silent" is rated for 12 dBA at idle.
Using PWM, these fans are able to spin as low as 300rpm, at which point they are going to be inaudible in all but the quietest rooms.
And Noctua's PWM driver IC
really works. These are the only PWM fans I have used which do not produce annoying noises at low RPMs. (ticking or whining for example) At low RPMs, nothing compares to Noctua fans.
Previously I would have chosen higher RPM voltage-controlled fans over any other PWM fan due to the annoying noises that they produce.
I think at some point, I will move to networked storage and build a PC using all passive components simply as a media player as Mfusick suggested, but I'd fit those fans on the intakes/exhaust of the case.
You won't hear them running, and they will help prolong the life of your hardware - components like motherboards may be fanless, but they are not designed to be passively cooled - it's expected that there is airflow moving over them from your case fans/cpu fans.
Originally Posted by Mfusick
They are almost always 80mm standard fans.
120mm is standard on modern cases, though Noctua do have a PWM controlled 80mm fan as well: http://noctua.at/main.php?show=productview&products_id=45&lng=en&set=1