I use a newer Haswell i3 NUC as a HTPC, and it works fine. As for software, I use Windows 7 with WMC and PLEX. WMC is just for TV via a Ceton InfiniTV 6 ETH tuner. PLEX handles all of my shared media that's on my file server, and it puts a menu option in WMC so you can easily launch the software (you can do this with XBMC via free third-party software). If you open PLEX using that menu option in WMC, WMC will reopen when you close PLEX.
I'm not 100% sure, but you might just be able to use PLEX without entering WMC. I'm wondering what would happen if you started recording something in WMC, closed WMC (it will still record), and then opened the WTV file in PLEX. WMC will constantly pull in new data while you're watching the video, but I don't know if PLEX will.
EDIT:
As for hardware, I use...
RAM: G.Skill Ripjaws 8GB (2x4GB) SO-DIMM - This set was on sale for $50 in December, which is a fantastic deal.
SSD: Crucial M500 120GB mSATA SSD - I didn't think that I needed
that much room. I probably would've been comfortable with about 90GB, but I wasn't keen on having 60GB. Some media programs like to store or cache metadata files locally. My PLEX folder uses around 10GB!
WAN: Intel 7260HMW 802.11ac + BT 4.0 - I don't really use WiFi on any of my set-top devices unless I absolutely have to (like on the Wii U), but I mostly got this because I need Bluetooth for my
Logitech MediaBoard Pro. By using the internal card, I don't have to sacrifice one of the four USB ports for a Bluetooth adapter. As a note, that's a really nice keyboard for HTPC purposes. My only complaint is that it lacks a Windows key since it's designed for a PS3.