If you are going w/ a XBMC distro I would recommend an Intel manufactered board like the DH57JG or at the very least a board with Intel network cards. My favorite distro for XBMC is Gentoo though Arch is a good middle ground between Gentoo and Debian.
For your poor WHS speeds what is 1- your switch do you have; 2- your network cards; 3- what drives do you have in your WHS?
