One thing as well, you have to look at, is HOW you will be playing back, etc.
If the HTPC will be doing all.. or will you have a separate server/nas hosting the media, and the HTPC for just playback.
A NAS is usually fairly low energy consumption, so that is a bonus.. but you have to look at SPECIFIC NAS's which run/host specific media servers or DLNA depending on how you want to view.
Other option you can do, is use your HTPC, as a server as well. Little more power consumption, as its always online, etc.. but can act as a server as well as player, etc.
EG: I have my server (small web host, etc) which is always on. I have all my media on there, running PLEX server. I have my TV upstairs with a built in plex player, and my HTPC downstairs for playback.
But prior, had the media server running on the same HTPC.. worked fine.. but didn't want to always run 2 boxes all day anymore.
Rogers Ignite Xi6, LG 47LV5400 LED TV, Yamaha