The build looks pretty good, but there are a few changes that could be made. What is the intended use of your HTPC (1080 HD playback, BluRay capability, TV recording and playback, etc...)? You don't need the separate GPU since the 780G's integrated GPU can play 1080p content. This is the system I would recommend:
It's main use will be to play back 1080p video files (.mkv, .wmv etc) to my Samsung 52" 1080p LCD as well as output audio to my 7.1 home theater receiver.
Issues I've had with on-board video before was the video would get out of sync with the audio, although I purchased the coreavc licenced and it fixed it, but also put in a 7600gt in the old machine too, so not sure what fixed it, guess I was thinking better safe than sorry.
But if you feel that Gigabyte board will be able to output the audio and video perfectly fine and support 1920x1080 resolution, I'm all for it.