I am also interested in this and I have found a really good article on building your own pc media center. Its the cheaper way to go right now rather than buying a ready-made system.
You can find some good "how to build" instructions here if you are interested in making your own
I've viewed one before and I may have just been paranoid, but the graphics felt like they hurt my eyes. Before that I was all about it, especially because I used to live in a small apartment, and could have used the space saver.
