In building a new PC, I was obsessed by the quest for silence. But airflow is needed to keep components from overheating. I selected components with wattage in mind...fewer watts means less heat to expel. My new Ivy Bridge PC with SSD will idle at well under 40 watts (that number rises under load, but I think it will be okay), so I figured I could get away with the quietest fans. Comparing fans is like comparing beer brands; everybody's got a favorite, and "quiet" is somewhat of a subjective evaluation. I've been happy with Noctua; their fans seem to last (6-year warranty!) and are quiet to my ears. So I got this case, a Antec Sonata Series SOLO II Black ATX Mid Tower Computer Case, and installed two inlet fans in the front, and also replaced the stock fan in the back, which was already pretty quiet. (That's a total of 3 case fans, 2 in and 1 out, to keep air moving through. I figure several silent low-flow fans is better than fewer noisy, more powerful fans.) I'm using the ULN adapter, which is the slowest, quietest setting here. The system is powered by a Seasonic 400W 80 Plus Gold Fanless ATX12V/EPS12V Power Supply - X-400/SS-400FL, which vents at the top of the Solo II. The result is almost total silence, although you can sort of feel that the system is running if there are no other noises. If you're pushing more watts into your box, you might want to take one step up the ladder to a fan that will move more air, such as Noctua NF-S12B FLX 120 x 25mm Fan (600/900/1200 RPM). Or read other opinions; some people prefer the Nexus 120mm Real Silent Case Fan - D12SL-12. Just check your temperatures to make sure you haven't gone too far.
I'm giving this fan 5 stars because, for what it is, it's the best. I'm not going to quibble over saving a few measly bucks; this fan does what I want, and it's perfectly affordable.