Well here in the UK we can easily record 20+ DVB-T standard digital terrestrial TV channels concurrently while watching 2-3 channels live and or playing a ripped DVD. Our Cores are typically 3Ghz Celeron's with 512mb-1Gb RAM and are typically based on i945 class motherboards. With this kind of level of activity at the core we typically see less than 30% processor usage. Obviously recording 30+ channels concurrently is not a likely scenario in normal domestic usage... but it gives you some metric on the capacity to load up a system.
Hope this helps
Which device(s) do you use for the reception of the DVB-T signal? (If you don't mind my asking)