IMO get a cheap ASUS mobo, a cheap AMD athlon dual core, then a PSU, small HDD, 2GB RAM, OcUK's CoolerMaster Micro-ATX case and MediaPortal, full setup costs less than £220!
AND use a cheap passive HDMI equipped graphics card or integrated graphics with as much hardware support for HD content as possible, saves power, makes the unit quieter and gives better results that the cpu doing the work