The previous HTPC I built was only good for XBMC and NASbox (see http://pcpartpicker.com/b/M7y), which happened to be all I needed it for. As the years passed, my needs have changed. I now stream multimedia to my mobile devices with Plex, have many steam games that are controller compatible, and have a fiance that needs any type of multimedia at her fingertips. Updating the mobo, cpu, and having discrete graphics will hopefully give it the additional capability of light/medium gaming on the big screen, Plex serving/transcoding, and other miscellaneous multimedia playback, including 1080p youtube. I have broken down each category/component and why I chose it.
CPU: I struggled with this one, since I had not been able to find many people that had the same needs that I did. For streaming and gaming alone, I knew that a dual-core would be sufficient. But I had the added requirement of needing to transcode videos on the fly. Transcoding is very cpu-intensive, and I would be worried about whether a dual core would be sufficient. I also don't think it gets off-loaded to the GPU, so whether it was an APU or CPU didn't really matter. I went with AMD for the price, and definitely wanted 65W since I would be using the stock cooler. Was debating between the A10-6700 and the 760K + discrete R7 240 card. Went with the APU for the lower power consumption and really poor cost/performance of the R7 240.
**The first thing I do when I have a new cooler (even aftermarket) is to lap/sand it down to get a smooth surface.
RAM: was in a combo deal on newegg with the APU, but I needed something low-profile and fast. This fit the bill.
Motherboard: I had went with an mITX build 3 years ago when I first made my HTPC. I liked the compactness of it, and I actually switched to a larger case to allow the HTPC to be a NASbox as well. Naturally, I had to upgrade to an mITX mobo, also wanted 6 SATA ports available. There weren't many options, and the ASRock one had the best specs and reviews.
Storage: These were just acquired over the last 8 years as I have been upgrading and rebuilding my computers all the while. My old laptop harddrive isn't even in PCPartPicker's database, but was my OS harddrive on my old HTPC. That has since been replaced with the samsung 840 evo. I am putting the two 1TB drives in RAID 0, and leaving the others separate.
Case: I had been eyeing this one for awhile about a year ago. My first ever PC that I built 15 years ago was in a all-aluminum Lian Li case. Lian Li just has a sleekness about their cases that I love. I needed this one for the capability of storing many harddrives, and had no use for a 5.25" bay (this has none). This perfectly fit my needs. Cable management is a bit of a challenge in this case.
PSU: This was solely limited by the case choice. I had to research which ones would fit in a relatively small case, and it turns out this is one of them. I would have loved to get the modular version, but that one is slightly deeper, and would not have fit.
Keyboard: Amazon had their gold-box deal of logitech peripherals last month, and I thought it was time to have a dedicated input device for my HTPC. The previous HTPC was controlled via Unified remote and XBMC remote on my phone. Free-typing was a pain, until now!