You mentioned XBMC.... reading the beginning of your paragraph before you said XBMC I was actually going to suggest Plex Media Server, another great use for this home server build. I'm currently running Plex off my desktop and feeding a number of clients, and plan to migrate that function over to the NAS/server once I build. There's a Plex app for my Samsung TV, and there's plex for Apple TV, Roku, and tons of other devices... so cost of entry would be very low if you want to add some 'smart tv' functionality to your TV downstairs if it has none. I actually just set up "Rasplex" for my bedroom TV... which runs a Plex client off a $35 Raspberry Pi with a USB wireless N adapter. Just two plugs coming out of it: one going HDMI into the TV, and one USB which actually plugs into a USB port on the TV and sends enough power to not require a wall wart
Plex is forked off XBMC, but I think their media detection engine is pretty rock-solid compared to XBMC, and it's just as extensible. If you aren't already wired into XBMC, try them both out.
I also run an app called Serviio, solely for it's very simple DLNA sharing. There's DLNA support built into windows that's part of Media Player, but it sucks, creates too many subfolders when you browse from a DLNA device (such as a Samsung TV with 'AllShare')... but Serviio let's you control what categorization folders to expose to devices browsing. Basically, I run it as a backup to Plex.... if Plex has some issue/crashes/is slow for some reason, rather than frustrating my fiance while trying to troubleshoot, I just cut over to using the DLNA client built into the TV instead of the Plex app. I have it configured to point to the very same folders Plex monitors for media. I don't use it often but its a nice backup to have around. Any DLNA device you could plug into your TV would support this.
Re: shutting down, etc --- you could configure all your network devices for Wake-On-LAN, that's one option. Other option is just enable Windows' Hybrid Sleep feature... it sips very little power and is pretty much instant-on. Windows 7 and 8 both support very low-power sleep states. I know unRAID is smart about spinning your disks down when idle, but Windows can be configured to do the same, and I think FlexRAID even has some power management settings of its own to let you get more granular.
I could keep making recommendations all night
If you're a uTorrent user, I'm also doing some really cool stuff with an app called FileBot, which has allowed me to set up a fully automated TV show downloader/file extractor/file renamer/file copier process. As a bonus, after it finishes running its magic, it even phones up my Plex instance and triggers him to rescan his library for new media