HD Satelite channels are usually broadcast as DVB-S2 which narrows the choices somewhat. FireDTV cards are highly regarded and well supported in MP.
I believe the latest plan is for some HD channels on terrestrial Freeview, this will be in DVB-T2 and MPEG-4 so new tuners will be needed.
see here:
http://www.digitalspy.co.uk/digitalt...w-hd-plan.html
and here:
http://www.digitalspy.co.uk/digitalt...-freeview.html