OK, back to the rest of the questions:
1) Think of the HTPC as a preamp with perks.
3) Haven't a clue, but you're probably going to be back to recording by time/channel, not program.
4) Nope, you need a video card that supports HDCP--HD Copy Protection. I think that locks you to HDMI. Incidentally, your TV also has to support HDCP, which I'm sure it does, since it's brand new. The OS also must support HDCP--that means Vista, not XP, and very, very likely not Linux.
5) Important, especially if you're doing HD processing. If you're just using it as a DVD player and MP3 player, not so important. Big hard drive? Veeeerrry important. And with that hard drive comes risk. How much are you willing to assume? Do you need to consider backup? I would certainly consider redundancy, and that means RAID--and not that piddly RAID 0 crap. That's 2x as dangerous as a standalone HD.
6) Yes. But not for HDCP enabled content.
Fairly tangential to your questions, but it could be an interesting read anyway:
Cable Card Primer
