Stereo receivers don't have decoders for multi-channel formats (although I think a few of them have Dolby Virtual Surround), but that doesn't matter because generally speaking your player equipment will have those multi-channel decoders. For example, a DVD player has a stereo output, so there's your decoder. The receiver is then just providing "everything else" (volume control, amplification, etc).
Power consumption is tricky - with modern AVRs most of your power demand is from the DSPs until you push them to insane output levels (many modern AVRs will "idle" at 100W or more), while most stereo receivers are "dumb" by comparison, and all you're providing for is the amplifier's needs and whatever basic microcontroller runs the device (things like the remote control receiver). Older AVRs can actually be fairly good power consumption wise (close enough to a stereo receiver to not really worry about it, relative to the extra features); I'm not sure where the very cheapest but very modern AVRs fit in (I would say as they increase in DSP features and similar, they will increase power demand). Anyways, at most we're talking around 100W (even for normal-ish output levels, at least in stereo), which is not extreme (and it's not like you intend to run the thing 24x7, right?).
A lot of the power consumption stuff is based on measurements of my own equipment and other forum posts.