I guess the hardware requirement(s) answer would be, check on Ubuntu and MythTV.
If this thing is going to take off, a few choices for minimum level hardware pieces is absolutely imperative. I am a Windows Media Center fanatic, but I'm ready to give this a try right now. But... Only once the powers that be who want to put this application on top will compile a recommended hardware list. I can't/won't spend time and money to find out that a capture card won't work, or that a video card has crappy driver support.
You may rename this thread "Desperately Seeking Recommended Hardware" if you like.
Maybe I'm missing WHERE the recommended-hardware/minimum-hardware requirement list is located?
Also, playing High Def usually takes more resources than 480p or 480i. I'd like an idea as to what kind of CPU/Memory will be needed.