Running xbmc through the openelec operating system is also a good option. Though I'm not sure how well any of the options mentioned work with wmc, I dont use wmc myself. The minix line are supposed to be good, but I would stay away from android software running xbmc. It's getting better, they all seem to suffer from one issue or another.
OpenElec on the other hand is great. I just replaced a pivos xios ds, with an Intel Nuc i3, with 4gb ram and a 60gb hard drive. It came with a copy of Windows 7, but for now I'm just using openelec. It boots straight to xbmc in 10 seconds or less, plays every file I've thrown at it flawlessly, even 3d. Wish I would have went this route a long time ago. The difference from the pivos box is amazing