At this date I would recommend a motherboard that supports the new Intel Core 2 Duo -- currently the best thruput/power/video performer. Also there are now a few Nvidia video cards that support HDMI. After a couple more months there will be more and I would surely want to consider using one of those for hi def video playback (esp HD DVD or BD shiny discs).
http://www.globalcomputer.com/applic...=CP2-DUO-E6600
HTPC's are currently in a state of technology transition. There are now reports of some people having both a HD DVD drive (pulled from a HD-A1) and a BD drive in a desktop PC with a HDMI video card and able to playback high definition in both formats using WinDVD8 (purchased from a Japanese website). The new Mandatory Managed Copy (MMC) soon to be final approved in the AACS content protection spec will allow you to create a hi def media server with storage on hard disk. 500GB USB2 or Firewire external hard disk drives would work well.
Bob