CPU passive cooling

[jimmy29]

The Athlon X2's are a very good choice for low power but capable systems. You should be able to run one passive with that heatsink, with no reduction in clock speed and it would handle HD with ease.

 

[alex24]

You could always buy a really quiet fan to go with the heatsink and voltmod it to 7v or something, I doubt you'd be able to hear it with the sides on the case, plus you'd have some airflow to stop the heat building up too much.

 

[EsaT]

Thermalright HR-01 Plus and Noctua NH-U12P are better choises for passive cooling. TRUE suffers from too tight fin spacing already with low speed fan.
http://www.silentpcreview.com/article850-page5.html

Also if it will be always on you should definitely look for motherboard made for power efficient PCs for actually being able to have low consumption. Also it's better to have it go to sleep mode when not in use.
http://www.silentpcreview.com/section14.html

The Athlon X2's are a very good choice for low power

Lower end X2s can match power efficiency of Intels (not performance) but high end X2s are plain horrible power hogs.
http://www.xbitlabs.com/articles/cpu/display/dualcore-shootout_8.html#sect0