VDR Software Decoder Plugin
Historically, VDR required an AV711x based DVB PCI card to decode Audio and Video and to render it's OSD. Since these cards are quite expensive and old-fashioned now, so called budget cards dominate the market. They deliver the MPEG signal to the software and it's the main CPU that decodes it. See also Output devices (VDR wiki) for information.
As far we know there are two alternatives: the Xine-Plugin and the VDR Software Decoder Plugin --- see above link for more info.
The VDR Software Decoder Plugin is even able to decode HDTV channels, and avoids the limitations and problems of the AV711x based PCI cards. However, this plugin is not installed by default, so you have to expect some amount of work to get a software-decoder based VDR setup running.
Sadly, no portable OpenGL-based renderer is known which would allow to build a STB with any cheap modern graphics card. (is this true, allows the xine-plugin OpenGL outputs?)
I can only speak about the softdevice plugin. It works very well and with any graphics card that supports Xvideo. No 3D-stuff required. For sound it uses Alsa but here I had to use Alsa's dmix plugin else the sound stutters.