Cool stuff for libav.
With regards to the build, what's your budget, primary uses, and monitor setup? I also don't know what case you picked, but it's worth investing in a good case. They will last several builds and make building rigs infinitely less painful in a well designed one. The initial build list looks acceptable without knowing the answers to the aforementioned questions, but I would split the single 3TB hard drive to smaller OS+App+Game drive (maybe an SSD, or at least something high performance like WD Caviar Black) and a data drive. 3TB is a lot of eggs to put in one basket and it makes imaging backups a pain. Splitting data out lets you run regular backups on just data rather than all the cruft in the main drive.
And I would definitely spring for the extra 8GB of RAM. There's literally never a good reason not to get more RAM if you can afford it. I'm definitely getting a minimum of 16GB when I upgrade my machine.