I've got an Acer Aspire Revo R3610, which is small and neat. I'm currently using it as a Linux webserver. I don't know what the HD video performance is like, but it has ION. If you're streaming, you're probably not so bothered about that.

Until this weekend, if I'd wanted a small media box (and wasn't bothered about the fact that storage is not expandable), I'd probably have opted for the R3700, but I've moved away from Intel Atom. Read on...

However, I've just (this weekend) built an AMD E-350-based system (using one of these boards), and I think that the AMD chips are the way to go right now.

Aside: I've got mine built out as a NAS box, but I guess you're looking for something smaller, in which case the Zotac stuff looks pretty good.