I am actually planning on having some transcoding on the fly out to the PS3, so a bit of grunt is necessary.
That's different. A low-end Athlon or C2D would be useful then.
Connecting the server to a monitor and keyboard during boot won't be a problem, I have both nearby if necessary; I only refer to it as headless because that's how it will be during normal operation.
I'd still go with a board that permits remote operation.
ECC memory intrigues me - is it really worth paying extra for?
You will get differing opinions. I think it is worth it, since memory errors slowly can corrupt a system, particularly if you build from source. RAM is so inexpensive these days that I personally don't think it is a big deal.
What does change is the motherboard and its components. Most consumer boards don't allow for its use, and even if they claim they support it, well, often they don't. You really need to move into classical workstation or server boards to get good support. Those use Opterons or Xeons, and they also are much better made, and include quality components (like Intel or Broadcom NICs -- none of the RealTek nonsense). The cases too usually have hot-swap drives, though that's probably not really necessary for your box.
That's pretty much the route that I've gone in the past, but now I'd rather buy some new parts and build something from scratch - everything secondhand I have access to is pretty much on it's last legs.
There does come that time, and I'm reaching it myself. Still, my main box is an early dual Athlon that is about seven years old, and it is fine.
One thing you may want to consider is a used pre-packaged, low-end server from HP or Dell. I too like to build my own, but for a simple system like you are proposing it often is more cost-effective. While there is nothing wrong with using a consumer desktop as a server, I have a strong bias against it. They are just not as well made, cut every corner possible, and are not really suited for the task.