Quiet!
For those who are interested in such things, I have a quiet computer. I mean really quiet. The sound of the hard drive is clearly heard above the fans, especially on power up. There are 120 mm case and CPU fans, controlled by the motherboard. The fool thing used to sound like a jet. I was using a Coolmax V-500 power supply. I like Coolmax. Their stuff lasts a good long time, and is quite reliable. I purchased a ThermalTake TR2 500 Watt power supply. (There are several TR2 models, the 500W is what you want.) The thing about this power supply is that it has a variable speed POWER SUPPLY fan. The fans controlled by the motherboard are variable speed, but the big, honkin' heat-sink on the CPU means that they barely spin to keep things at a nice temperature. The power supply fan is usually going full bore no matter what the computer is doing. I spent some time looking, this looks like it is unique in it's price range ($60 from New Egg, I bought it for less, and I got a rebate.) There are a few fanless power supplies, but they are well over $100. If you want a truly quiet computer, check out the Thermalkate TR2 500W (WO379RU) power supply. It has a five year warranty, I have no idea how long it will really last, but at this sound level, I would buy it again and again even if it only lasted a year or so.