I am planning on using the free, open source MythTV software to act as my media center and I plan on running it on Ubuntu using the MythBuntu distribution so my hardware selections factored in compatibility with Linux. The hardware below should also perform well if you plan on using Windows Media Center.
In future posts I'll go into more detail about the hardware and how to put everything together but bellow is my HTPC hardware requirements and the hardware I chose to meet those requirements.