How big do you want it? It could be easily done with a Itx or ATX standard build
(Left to right: ATX, Micro-Atx, Mini-Itx)
You do realise hdds are noisy so having them in the same room as your top of the line TV and sound system is fairly detrimental. A small drive on site (SSD or Samsung 2.5") with large storage hidden on a server in a cupboard or attic such as a Pentium 4 build which can be picked up second hand for very little.
The above gfx card suggestion is rather idiotic as it has a fan and all Ati 4000 series support hdmi same with the 8 series and up for nvidia. Not that it matters however as why would you want to interlace the audio with the video if you are using a dedicated optical line for the audio? Its a needless extra.
2gb of ram will be sufficient maybe 3 if your planning on video encoding and light multimedia editing.