Having played with MythBuntu, I'm fairly sure that that'll be the right thing to run on it. An Atom based Mini-ITX system looks to be the right mix between power use, features, cost and size. Since it'll also be a real box running linux, I'll be able to move all the networking services onto it from my desktop. Couple that with some wake-on-lan magic that the desktop seems to support, and I should be able to put the desktop + screens to sleep when I'm out, ssh into the media box and wake it up the few times a week when I'm out but need to access it. All looking like a plan.
Next up, I need to pick the motherboard for the media pc. There seem to be three main choices. First, there's the current intel mini-itx atom motherboards, which come a really low power processor, but a really power hungry chipset, which does rather seem like a waste... Oh, and they don't seem to do dvi/hdmi, which I'll want for going into the TV. Then there's the new D945GSEJT from Intel, which has the new low power chipset, dvi support, but only a slower, single core atom. Oh, and it isn't for sale quite yet over here (it only came out in the states in the last week or two), but seems to go for about $120 over there. Finally, there's the NVidia ION atom motherboards, especially the Zotac Ion. These have DVI and HDMI, come with the dual core atom option, have wifi and a few more ports, but draw a bit more power (about the same as the older intel boards though - 21w single core, 25w dual core), and cost more (£140 for the single core, £180 for the dual core). They should be on sale over here in a week or so.
I figure the older intel boards are out, so it's a toss-up between the newer intel board, and the nvidia one. Do I want a slightly cheaper, slightly lower power, slightly less powerful intel one, or the the more powerful nvidia?
Oh, and then I need to pick a case, pick a harddisk for tv recording (can't just use my desktop as that'll soon be asleep much of the time) and decide between pci and usb digital tuners. Choices, choices....