I'm not a Media Center expert, but will take a stab at your questions.
1. Yes, things have changed. As of Vista Media Center, it now supports CableCard. This is OEM only, so you can't just go out and buy it, but it will allow your MC machine to decode HD for viewing on your TV. Hip hip hooray!
2. Sorry, I don't know that one. But since Media Center machines are OEM, you're going to get a machine with the right hardware in it to use Media Center like it should be used and enjoyed.
3. I don't know this one either, but if you're wondering if HD content can be streamed over wireless, the answer is yes. I believe Wireless-A is recommended, but Wireless-G is also supported.
4. No. Media Center Extenders (the Xbox 360 and others) do not record TV from a cable box themselves. They are simply an extender, so you can access everything your MC machine records, music, photos, video, etc.