FWIW, I set out to build a new low-power general purpose/NAS/TV server recently. I went with an i3 because the idle power is so low, yet has enough oomph to do overnight transcodes of BR rips and the like. Oh, and transcoding .ts files to correct the errors (did I ever mention elsewhere how much I hate .ts? Not sure that I did ... :) ).
I paired it with a similar disc setup as Adam's for similar reasons: an old 2.5" boot/scratch/temp/PVR drive, 3x3Tb WD Red drives configured as RAID-Z (ZFS) for fault tolerance. I stuck with USB tuners because they were what I had, although internal cards would have been tidier/better (no device numbering issues on boot).
I went micro ATX because of the SATA ports; I was looking at mini ITX before that, but there was only an ASUS board that came close. I've got the room to add another few drives as needed now, though.
Major parts:
* Intel i3-3225 (I think the only 4-core i3)
* MSI H77MA-G43 motherboard (seemed to be the lowest power uATX I could get)
* Antec 380W EarthWatts Green PSU (80Plus Bronze - nothing is really efficient at low power unless you go picoPSU)
* 8Gb Crucial RAM (single stick, takes less power - 8Gb because I was considering running Squid, although I also have the space for a decent RAMdisc if needed)
* WD Red Drives (so they can spin down independently)
... plus the inevitable bits and bobs. I think it consumes about 25W on idle, peaking at 70-80W when all four cores are active. The upcoming Intel chips would lower this further, but I couldn't wait any longer; I also considered a U processor, but decided that it just didn't have the grunt for very little return (lower TDP, yes, but not lower idle power or when only a single core or so is active).