I'm going to have to look more in to mythweb.
But as far as using your computer as a Tivo.. t may cost moret han 700$ :P
You may chose that 400$ computer, but it's a different story for TV tuners. First of all, if you're planning on watching TV through your computer, or recording for that purpose, you're going to have to have some good TV tuners.
I bought one, I used it for a while and it blew. The quality was horrible. I sent it back, I got one that costed 150$ and the quality still isn't as good as I want. If anyone has a solution, post it. I'm using WinTv.
Now for the recording.. I see Mythweb can set up a scheduel (this is what i'm looking for for my girlfriends mom). I wanted to build a computer that was simply for recording. I would need 3 dual tuners for this computer, because most of the time she records 6 shows simutaniously (they all come on at night). Now, I said 150$ didn't even get me a good quality one.. now imagine more times 3. I looked it up not too long ago and I saw something about a quad tuner.. was expensive.
I don't know if this is along the same lines as what you wanted.
Alss, my girlfriends parents just got a 50" plasma. I hooked up their computer to it, which was bought last year (just to give you an idea of the parts) and was about 1k$. The video card couldn't handle 50" very well... I hook mine up and it was great.
Coudn't handle it well meaning it would lag and couldn't play the bluray movies I put on it to test very well either. I figured it was just the parts, but it's not. It's the card.
Just a heads up! I built my comupter for almost the same reason, I think, that you are. Yes. I can watch TV on it good enough and record. There are some questions that haven't been answered online about TV tuners... I looked for days and asked and no one knew. I finally emailed Hauppauge and they're awesome... they helped me with ALL of my problems. So... hauppauge is a good brand
Plus alot of people stand by it.
Disreguard this post if it's not what you're looking for,