Sensei George passes on a link to a good how-to article on building a home theater based on a PC. Specifically, one dedicated entirely to living-room use.

And this is probably the best advice if you decide you want to build (or buy) one of these - don't use it for anything else, or you won't end up using it as a media center. Certainly our Media Center PC is pretty much always in use by the kids for "other purposes", which precludes it's use for Media Center tasks.

The link:

Building Your Home Theater Around the PC (Tom's Hardware Guide)