OCC has published a new article on designing and building an HTPC
"The majority of you already know what goes into a computer, but there are a few extra considerations to make when it comes to building an HTPC that you might not normally think about. Things like noise and temperature begin to play a huge part in its operation, as well as physical size. When it comes to the components, you first need to examine what your use for this computer will be. Some may be building a brand new system, but you may be constrained by using spare parts you already own and coupling them with a few new ones to complete the system. Keep in mind you might be limited in functionality depending on the components you choose. Some of the typical uses of an HTPC are listed below, and depending on your needs this will greatly affect the cost and components of your project."