Some time ago, the GPU acceleration in Linux, has been made a huge step forward. All you need is a moderately new kernel, 3.10 or higher, at least Mesa 9.1 and TV tuner with HD support and last but not least, a supported GPU device.
In an older post, we described the successful setup of a DVB-S2 environment for watching HD TV with a single board computer. The setup with an off-the-shelf mini-PC is very similar, especially since our tuner is a USB device. The GPU should be also no problem because even older cards usually have a profile for H.264 decoding. In our case, the tricky part was to find a compatible pre-packaged driver for our GPU device with the required interface for the acceleration.
After we glued everything together, we made a first test. The playback of a recorded DVB-S2 stream worked without any problems. The CPU usage was very moderate and the audio, video streams were in sync.
Since we are interested in a lightweight platform, we decided to focus on gnutv+mplayer for recording and playback. However, since mplayer does not seem to support H.264 for the DVB mode, at least not in our setup, we needed to combine both programs to watch HD TV. In this setup, some fine tuning was required to synchronize audio/video. In our case, the problem could be solved with a larger buffer (cache).