HD TV Tuner card query...

I am looking at putting together a pc media center primarily to record HDTV and I had a question about TV tuner cards.

If I wish to have the option of being able to record two HD channels at the same time (or indeed to record one HD channel and watch another HD channel at the same time) what are my options?

Do I need to purchase two separate tuner cards or do dual HD cards exist.

I see that there are dual cards which say HD/anologue in their descriptions but I have to assume that is not exactly what I am talking about here.

Thanks for your help.