The power supply supllies energy to all components of your computer. Therefore depends the maximum power that it has to supply from the components that you have build in.
If you use an economically VIA Epia Mainboard with only one DVB card and only one disc drive a 60 Watt power suplly should be sufficent. If you have such a low power device, it should be possible to cool it without fans and avoid noise.
If you use additional equipment, you need more power. A rough estimation is
- per DVB card ~20Watt
- per hard disc ~20Watt
The hard disc takes only so much power during power on as it spins up. After that the power consumption is normally reduced.
Nower days passive cooled power supplies with up to 300 Watts are available, may be you should think about this possibility. But they are expensive too, and then you may need additional fans for the case itself. If your processor or other cards gets too hot, they may have reduced life time and stability.