This is my HTPC I built about a year and a half ago. I just decided to add a video card so I may as well document it :)
CPU: The i3 is plenty fast for what I was using it for (XBMC video streaming, light gaming) and the HD Graphics 3000 served me well up to now.
Case: I wanted to use a case which looked good in the TV stand and had enough space for multiple HDDs and a standard ATX PSU. I've since moved most of the HDDs into a server. One of the case fans was broken upon arrival so I decided to replace both with the Fractals. They were too noisy at full speed so I had to run them at 7V.
PSU: I know its overkill but the Seasonic was from my old gaming build. So much better than the previous CX430 which had coil whine.
GPU: Decided I'd like to do some more hardcore gaming on the couch! Picked up the 7850 as it was about the fastest card that would fit in the case. Most of the high end GPUs have the power connectors on the top which prevents the top panel from closing. Only tried it briefly so far, but it runs Tomb Raider 60fps at high settings :)
Finally with the new PSU and some fan tweaking, its dead silent! FYI I'm streaming videos from an 11TB media server.