I'm not sure specifically about WMV HD, but Intel's CPUs tend to be quicker at streaming encoding and decoding chores than Athlon 64s so that at least cuts your choices in half.
It seems like just about all of the micro-ATX motherboard include onboard video which may not be able to drive the high resolution output you need so you may have to add a video card. And micro-ATX isn't exactly a Small Form Factor PC. Have you considered just getting a SFF barebones machine like
this or do you want to roll your own? If you want to build from scratch then you could start with something like
this ASUS P5GD1-VM. But even micro-ATX is going to be way bigger than a SFF PC and micro-ATX motherboards are definitely slot-challenged.