By "HD" do we mean video? In the case of the RX-51, decoding is going to be handled DSP-side (430MHz C64x+). I dunno about the Archos, but the Beagle has been doing 720p with NEON-only optimizations (ARM-side) since forever. Bigger than the SoC power consumption-wise, though, are the screen (5" is a lot more backlight to drive) and hard drive.