Can anyone recommend an off the shelf Mythbuntu ready system? Or as close to off the shelf as possible?

What I've got in mind is something that:
Runs very cool and quiet (lounge use. some atom based job? Perhaps with an air cooled external psu brick?)
Is fairly compact
Has at least 2x 3.5 internal drive bays (I want to use Raid 1 and also make them available as a basic home nas)
1080 HDMI output, and reasonable audio output
Has at least one expansion card space for a tv tuner
Has enough grunt to record at least 2 Freeview HD streams and play back a third.

Any ideas?