I like the mini ITX idea but for a mythbuntu home entertainment the flash memory is way not enough... You would want to have at least 250-500 Gb HDD, I was thinking of getting a 1Tb HDD
I was assuming that this would be a client streaming content from a server somewhere on the network.
In that case, add ~10 watts to your total energy consumption with a traditional disc drive (most will be less than that, but assume 10 as a maximum). You could go with a more energy efficient laptop HDD, but the trade off there is going to be with speed and capacity.
If you are going to have a large drive in there, I would still install the OS onto say a 4 GB flash drive, then use the HDD simply as media storage. This way you can power down the HDD when it isn't being actively used. If you intend to run this media center 24/7 it is going to save energy in the long run to just spin up the drive whenever you request a file off of it and run out of flash the rest of the time.