What to know when building a HTPC

I read a post earlier by someone looking to build a HTPC, which sparked my curiosity. I've been doing some research but can't really find a clear answer to my questions. So out of pure curiosity...

I could see how a pc could replace most of my home theatre, but what about my digital cable, is that possible? If not, is there some other way to link the digital cable reciever to the pc to give me full control of my enterainment system through my pc?

I've seen the tuner cards, but any forum where I read about them gives mixed opinions on whether they work for HD digital cable. Also, I live in Canada, so any solution would have to work with a Canadian cable provider.