Ok, so I just built my first computer dedicated to media use. Now, before I go on, you should know that this machine was built out of extremely old spare parts I had lying around. Thus, it's not a cutting edge technology. That being said, it does some cool stuff. Here's the specs:
Pentium 4 1.6ghz
512mb SDRAM PC133
7200rpm 40gb Maxtor HDD (At least 8 years old)
500gb Seagate external HDD
64mb ATI Radeon 9000 AGP video card
Windows XP sp3
The point of this build was to be able to play music and movies on my Television. More than that, though, I wanted to be able to stream online content (i.e. Netflix or Hulu) to my television without the need for major geek intervention. This is not a true HTPC (Home Theater PC), for it lacks the capability to record/playback live TV and the ability to reproduce digital surround audio tracks.
For streaming online content, I've found the beta software Zinc (made by ZeeVee) to be very useful. It's basically a stripped down version of Mozilla Firefox designed for content streaming. It has plug-ins that work for a host of channels, including ABC, Fox, CNN, Netflix, Hulu, etc. It is also capable of receiving streams from media websites that host RSS feeds, such as ABCfamily.com
For local media (i.e. videos stored on the hard drive) I find it much easier to run Boxee. Boxee is based on the popular media center software XBMC (Xbox Media Center) but has completely rebuilt the interface. It too has the capability of streaming web content, though it is much more limited, and it has music support for Pandora and LastFM. When playing music from internet sources, it will show track and artist information inside of a full screen visualization software. Boxee is still in Alpha testing stage, and I'm looking forward to testing its upcoming releases.