The first question of course is if it's worth spending hundreds of dollars to build a more efficient PC if it's not really going to affect electric usage much even over the long term.
In the meantime, let's discuss the build considerations a bit.
With the understanding that it needs sufficient computing power to perform those functions, the remaining key factors of the build would be:
- Energy Efficiency
Acoustic Silence
Small form factor
The GPU is subject to the monitor being powered and the media/content being utilized. (i.e. does it need HDCP support? will it be upscaling and converting content? what resolution is the display?) In my case, the existing monitor it would be connected to initially has a display resolution of 1680x1050, though there is a possibility down the line of something as large as 2650x1600, which would require Dual DVI capability on the GPU selected.
The speakers in my case are 5.1 Logitech Z-680s (DTS, DDL, etc). I currently run an Auzentech XPlosion 7.1 DTS card with the ability to connect it to the Z-680s via digital S/PDIF or optical Toslink. With smaller form factor it's likely the onboard sound will need to be used, even though that generally sacrifices some CPU cycles for sound processing that a peripheral card would handle itself. Given that movie playing is a primary use for the machine, 5.1 sound and the ability to process DTS and DDL is a must. If the onboard sound cannot do this, the motherboard and the case must be appropriate to fit a PCI (or soon PCI-E) Auzentech or Creative soundcard.
The case size should be determined by the components selected, and should not dictate the build itself, else it would force a compromise on the other primary goals of energy efficiency and silence. Some worthwhile considerations would be the number and location of fans and the airflow path in the case.
The CPU is probably the place to start, because that will determine the motherboard selection.
The RAM selection is essentially one of the last things to determine, as it is determined by motherboard compatibility first and foremost. Capacity would be a minimum of 2GB for smooth sailing regardless of OS selected.
The media drive(s) (CD, DVD, BluRay) are also a low priority choice since they are the same regardless of AMD or Intel, AMD or NVidia, etc, and are driven purely by performance and acoustics.
The power supply should not be a difficult matter either, considering this is not a multi-CPU or multi-GPU build with high-end gaming performance parts. The energy efficiency goal means that any 400-500 watt PSU should easily suffice, with the primary goals for PSU selection being energy efficiency and acoustics. For acoustics it is often better to get slightly more PSU than needed so that it is running at a lower percentage of max capacity and thus won't be as hot and won't need to speed up its fan.
What other considerations or parts would be worth discussing?