HTPC + server OS (1 Viewer)

So, in google searches i stumbled here, and a lot of results brought me back, so i figured id post up my question for the experts here.
what i have: a low power consumption 24/7 PC (AMD tricore APU, 3tb storage, 4gb ram).
what i want: double the PC as both server (more of a NASish setup, storing all my files, accessing remotely, etc) and HTPC. the computer will just be directly connected via HDMI to my living area TV and also an output to my main PC monitor that i can swap inputs to control.
question: what OS would handle both of these operations in a clean fashion? my first instinct was to fire up a fresh Windows 7 and use either Plex or just WMC for the HT part and deal with the (lack of) server part as more of a "file sharing" setup, but thats cheating for wanting an actual server setup. XBMC looked ok, but i quit messing with linux as far back as Backtrack 2.0 and knoppix live disks. my last thought was Windows home server, but i dont know the capabilities of the HT side of a server oriented OS.

Well, the choice of what you use for a media front end really depends on the goal.. I mean, if you intend to add CableCard or encrypted channel viewing to your mix, then WMC is the only thing on the market. But you need to know more about what your goal is.. what kind of media do you anticipate watching most?