I've got some figures on a mythbackend system that I will probably port to linuxmce on the next mce release.
http://mythtv.org/wiki/index.php/User:BlackoperI have a p3 kill-a-watt if you want any other power figures on any of my other motherboards/hardware
I got my backend down to 41 watts total power usage with the system 2.5" drive before adding pci and pci-e cards and hard drives
I use hdparm to spindown my drives and have no problem doing so. I give it two hours before they spin down to keep the bearings from being worn needlessly. I leave two drives active that are what myth records to. For some reason they won't spin down even if you want them to.
this is the section I put together on how to spin down hard drives for myth. Not sure if it will work for linuxmce yet though as I haven't tried it
http://mythtv.org/wiki/index.php/RAID#Spinning_down_hard_drives