In most cases, the tuner card/dongle will give you more or less the same reception as your regular TV will pick up.....note that there are some tuners on the market with Diversity feature which allow combining of two tuners to give a better result.....
I have a Vaio XL-201 MCE machine, which I have updated slightly to include a total of 3 DVB-T capable tuners etc.......this serves as our main TV/DVR and is excellent.....never looked back, as this PC has replaced the hifi, DVD recorder, freeview box etc., and also gives you a full-on network media player and 'browsing-from-the-sofa' thing.....