I am now confident recording DVB-T MPEG4 on a relatively low spec PC eg my P4 2.4
A further update and caution advisory
I currently record analogue SD as MPEG4 as it send my P4 2.4 CPU to 80-100% CPU depending which codec I use. The M$ MPEG-4 codec definately uses less CPU than the DivX or XviD MPEG-4 codecs [the later 2 send the PC to 100%]
Now I suspect that there is extra overhead in converting analogue to MPEG-4 as opposed to recording a DVB-T H.264 signal but until I get my DVB-T adapter I cannot confirm but I note that Haupaggue and Avermedia state a minimum of a Pentium 2.4 for recording DVB-T BUT as MPEG2. I'm not sure if the CPU for recording DVB-T as MPEG4 [H.264] will be more than with MPEG2 since the signal is already digital and unlike Anlogue does not require intensive CPU to covert to Digital and then compress.