A friend of mine (jils) built a home theater pc with the goals of it being quiet and cost effective. Quiet computers are achieved by eliminating fans, which implies having low heat, which means low power.
With AMD CPUs there is a software program called cool n quiet, that will reduce the clock speed of the processor when it's not being used. On intel computers this is generally known as speedstep and doesn't usually require software.
He describes his home theater pc designed for watching HDTV on his 720P LCD rear projection tv like this:
based on an A64 3500+ Winchester CPU So it is not one of the super low-power jobs, but still much cooler than a P4 of that generation. http://en.wikipedia.org/wiki/Athlon_64#Winchester_.2890_nm_SOI.29
The Mobo chipset is an Nvidia 6150 with integrated Geforce graphics sharing the onboard RAM.
80 Gig Seagate Barracuda 7200.7 IDE HDD
300 Watt Antec PS
Avermedia analog Tuner Card
Dvico Fusion HD Tuner Card
He reports the following power usage without the software:
All Measurements in Watts. AMD Cool and Quiet NOT Enabled
S3 Standby: 6
High Load: 98 W (720p x.264 Playback inside of the MCE Interface).
With Cool & Quiet software running, the 3500+ clocks down to 1000Mhz and up from there depending on load.
IDLE: 69 Watts
Watching 640px XviD: 69 Watts (no extra CPU needed for viewing).
Load: 720p x.264 MKV: 90-96 Watts
So he saves 12 watts on idle, and a few watts when it's decoding hdtv. Also interesting to note that the system is effectively idling when it's playing a dvd, because the cpu is still fast enough at the lower clock speed so it doesn't need to increase the speed (and thus power)
The intel core 2 duo chips are very cood at power reduction (probably better) but they can be hurt by the chipset taking up a lot of energy.
The AMD chip jils chose was very cheap, and thus also there's implied environmental savings as it probably took less silicon to produce.
Note that letting his computer go to standby (from which windows media center can wake up to record shows) saves a tremendous amount of power and still provides instant on.
He mentions S3 state which is a lower power mode of standby.
Note that screensavers don't save energy! only having your computer turn off your monitor, hard disks or go to standby (best) does! hibernate is even better.
However if you have a AMD computer look in to cool n quiet to reduce your power usage while you are using your computer!