Having two different machines would be ideal; servers aren't exactly known for having good HTPC qualities and vice versa. The server should be out of sight (definitely out of earshot) and out of mind.
What orihS is getting at is that you don't NEED to do it that way. You can ALSO forget about the external/network storage and just store everything on the HTPC.
A really popular choice for HTPCs are Mac Minis. They're tiny but have enough oompf to stream hi-def movies. They also have 5.1 optical out and lots of USB ports to plug in external storage.
They're also not THAT expensive, well worth the end result IMO.
Atom isn't a bad idea although it will take a very long time for any power savings to pay for themselves and I would steer clear of ECS. Before buying anything though, it is important to nail down a few details such as the role(s) of the server and what you need out of it.