There are basically two ways to dissipate heat. Fans, heat sinks, or a combination of the two.
As mentioned in the Silent MythTV Introduction, there are really quiet solutions as well as truly silent.
Truly silent power supplies generally work by having huge heat sinks and no fan. By huge, I really mean huge. Good "truly silent" power supplies have a quarter or more of the heat sink actually sticking out the back end of the unit. They are really expensive (they start over $100) and some may view them as rather unsightly. The huge heat sink transfers heat from the AC to DC conversion out the back of the unit. If you have the budget or it, and also yearn for true silence, then this is the only way to go.
The "almost silent" power supplies that market themselves as silent are much different in design. These are usually much more budget friendly, and are available in a very wide price range. Sound output level usually, although not always, directly in line with how much they cost. More expensive silent power supplies make less noise. They work by having one or two very high efficiency fans that turn at slower speeds that normal power supply fans and/or are designed in such as way as to make less noise.
"Silent" power supplies that have fans are usually not really silent and the cheap ones ($30) may not even be very quiet. Look at the specifications in terms of sound decibel output to compare different models. Lower numbers mean they produce less noise.
Don't forget you need to have enough power to run your system. Look at your motherboard manual for a recommendation for how much wattage your power supply should have based on your CPU and graphics card selection. In some situations, you may be better served by getting a high end "silent" fan-based model rather than go with a low-end truly silent model. If you don’t have your motherboard yet, you can usually find the manual on the motherboard manufacturers’ website.
If you don’t have the desire or budget to try to silence all the noise sources in your MythTV system, the power supply is the component you should probably address. Even if the only "silent" component you get is the power supply, you will probably have eliminated the maximum sound possible for the money.
