In many systems, the power supply is the main source of noise, so minimizing power supply noise is very important.

Try and using a standard power supply that is more silent than the normal one like the Antec TruePower or the PC Power & Cooling Silencer. Such power supplies cost same as its competitive products, and are reasonably silent. A system that uses one of these power supplies can be quiet enough to be fit in a normal residential environment.

Secondly try and use a power supply that is particularly designed to minimize sound like those made by Nexus and Zalman. These power supplies cost a more than other standard power supplies, but generate only 18 dB at suitable and not much more under load. A system that uses one of these power supplies can be un-hearable in a normal residential atmosphere.

And, there are completely silent power supplies, without moving parts that use huge passive heatsinks or convective water cooling to dissipate heat.