I should be taking delivery of my Rasberry Pi sometime soon. Only cost me around £30. I'm going to have a play around with it and see if it peforms well as a dedicated A/V server. It is capable of high def video playback (via HDMI) and also has a 3.5mm audio jack. The O/S (Linux) boots from an SD card, but I should be able to connect a HDD via a USB hub.