Thanks to maturola's help, I've looked at what is required to build a HTPC.
While I could build one for about the price of a second hand Mini (though with better specs), it seems that the reason I'd favour one over a Mini (HD Audio) is not as straight-forward as it seems due to the fact that neither Plex nor XBMC support HD audio...
Is there another media centre software package that would support HD audio, preferably running on Linux instead of Windows?
What exactly are you looking when you say HD audio?
XBMC Support:
Dolby Digital Plus up to 7.1 48kHz 16bit
Dolby TrueHD up to 7.1 48kHz 24bit, also with interleaved AC3
DTS-HD up to 7.1 96kHz 24bit, also with DTS-96/24 core up to 6.1
Meridian Lossless Packing up to 5.1 176.4kHz 24bit
