Perhaps not too late to share my experience on building a high-quality HTPC for the purpose of video & video, but not for playing games. My current HTPC is based on the Asus M4A785-M mobo, Phenom II x4 965 3.4GHz CPU, 4GB RAM, AMD HD5670 GPU running Win8.1-64. These are all mid-end hardware of a few years old and inexpensive. I also has a Blackmagic Design Intensity Pro recording card. I overclock the CPU by 10%.
With this setup, I can play video and audio of the highest definition, best color and smoothest actions. By that I mean play files streamed from a server via the gigabit LAN, video of 1080/60p at up to 35 Mbps bitrate. This is the highest specs for Blueray disc. Audio up to 24/192. All using JRiver. But this is not achieved without careful design and configuration of the entire system, learned from long experience. After many failures! Here, without order, are a few key things I learned:
1) HTPC gets hot especially playing high bitrate video and only lots of fans can control things. But lots of fans, or small fans, are noisy and very disturbing. After trying 3 cases, the problem is solved by choosing a large case which allows me to install 2 120mm fans at the front, plus a water radiator fan at the rear. I picked the Cooler Master HAF XB EVO case. The second case with a smaller cooling capacity burned out the power supply after 6 months. Only the large fans with rubber isolation deliver enough cooling and still stay quiet.
2) The decidedly mid-end GPU is enough for the most demanding video running JRiver Red October HD (madVR, highest quality scaling). No need to go further unless you play games.
3) You must configure your madVR carefully. Learn and do this and do trials on your situation. To give you an example of the extra demand of madVR, when I play a high bitrate video say 25 Mbps using Cyberlink PowerDVD 13, CPU runs at 25%. But play same using JRiver madVR, CPU runs at 75%. But JRiver certainly delivers a better picture quality on colors and fast actions. The demand on the HTPC, for a given resolution, is based on video bitrate. So you must know the highest video bitrates of your library in order to design the optimal PC for it.
4) For best quality, connect from the PC to the AV amp digitally. Use the GPU HDMI out connection for video, then either HDMI or S/PDIF optical output (mobo) for audio. Video decoding is always done by the HTPC running JRiver, then fed to the AV amp or TV as uncompressed video. For audio you have a choice of either using JRiver to decode, or feed the undecoded audio as 'bitstream' to the AV amp for decoding. If you wish to use JRiver to simulate surround sound from 2-ch video, this can be done (under DSP) but then the output will be encoded by JRiver as Dolby Digital multi-channel and can only be output via your mobo's S/PDIF port. HDMI audio cannot be used.
5) Make sure you TV is calibrated and its various color controls configured well. MadVR disables your GPU software color controls, leaving you only color controls from madVR. madVR color controls are crude so a TV not accurately calibrated will deliver compromised colors that hard to fix.
6) If you are streaming high bitrate video (20Mbps - 35Mbps) from a server you absolutely must have gigabit LAN and try to get your server PC run Windows Server instead of desktop Windows. Windows Server has higher LAN performance due to the network software design.