If people are really serious about building a power efficient PC then you need to choose the right components. The most important component is the power supply (PSU). The efficiency of the PSU ultimately determines how much power the PC will draw from the A/C outlet. The more efficient, the less power. I recommend Seasonic or Corsiar (made by Seasonic) PSU because they are both efficient (up to 85%) and very quiet. An efficient PSU also wastes less electricity as heat so the inside of the PC will be cooler.
The next thing to consider is the CPU. Generally speaking, some AMD Athlon X2 CPUs uses less power than Intel C2D CPUs. However, under load the Athlon X2 CPU uses more power than the C2D CPU. Therefore to get the best power efficiency, you need to determine what your PC will be doing most of the time when it is on. Folding@Home requires CPU cycles, therefore if you want to pursue this noble cause then the C2D will use less power, thus saving money. However, if you want to setup a Torrent PC, then you will want a low power AMD Athlon X2 since it uses less power than the C2D when it is idling.
See my previous for power consumption while idling. Below is a link to a different of the same article from above:
http://www.xbitlabs.com/articles/cpu/display/amd-energy-efficient_8.html
Note that different tests can give slightly different results. Also note that the Athlon X2 3800+ EE is being compared against the more powerful C2D E6300.
Next up is the motherboard. Unfortunately, no one really checks for mobo power conumption probably because it would be too difficult to exactly measure it since you need the eliminate power consumption of all the other components. Therefore, I can only provide general recommendations.
1. Buy an m-ATX mobo. There will be less circuitry and slots compared to full size ATX mobo which means less power is wasted.
2. Skip the "premium" options if you don't need them like RAID, WiFi, etc.
3. Get a mobo with a passive chipset cooling solution, meaning just a heatsink with no fan. If you have a power efficient PSU, then there should be a little less heat inside the PC.
The sound card - While on-board sound has improved over the years, it is still inferior to a dedicate sound card, but should use less power.
Hard drive - If possible try and buy a hard drive that uses only one platter. That may be difficult depending on the capacity you are looking for. The fewer the platters the less power it will uses, the less heat will be generated and the less noise it will produce. I think at the moment the most a single platter can hold is 200GB. But Seagate recently announced that they will be releasing hard drives (late October) that uses platters capable of storing 334GB. I plan on getting the 1TB version.
CPU heatsink - If you build your PC correctly and you do not intend on overclocking, then you can opt to install just a heatsink to cool your CPU. Eliminating the fan saves power and reduces noise. Which passive heatsink you plan on using will depend on how hot the CPU gets and the side of the PC case. If it is really wide or tall, then you can install a massive heatsink like the Scythe Ninja.
The Video Card - As I stated before, the 7600GT does not use a lot of power when idling. Many mainstream and value GPUs use little power when idling. Choosing the right one will optimize both your needs and power consumption. Over at least the past 4 generations of GPUs, nVidia's cards have generally used less power than their ATI counterparts.
Any video card can handle DVD movies. However, HD-DVD or Blu-Ray is a different matter. Below is a link to an Anandtech article about HD video decoding and is worth a read:
http://www.anandtech.com/video/showdoc.aspx?i=3047
Like I said before, nVidia GPUs generally uses less power than ATI cards. However, ATI card are marginally better than nVidia cards at offloading HD decoding from the CPU. Given this I would still choose a nVidia card. But (to the best of my knowledge) current nVidia drivers doesn't allow the GeForce cards to do HD video decoding under Windows XP.
Another thing to consider for HD-DVD or Blu-Ray is that both GPU and monitor must have HDCP to see HD video in all it's glory.
If I had to choose a card right now and I do not intend on playing games, then I would get a nVidia 8400 with HDCP. It uses very little power, therefore you can easily find one with a passive cooling solution. Thus, saving some power on the fan and also have less noise.