Here’s a couple of weekend articles about home built HTPC's.

Coding Horror - Building Your Own Home Theater PC 
Ars Technica - Ars System Guide: HTPC edition

Both articles are good, but I still prefer to own a Digital CableCard tuner and that forces me to purchase a CableLabs certified system from an OEM like DELL or HP. What's a CableCard tuner? Basically an analog TV tuner will only allow you to receive basic cable, but a Digital CableCard tuner will allow you to receive (most) Digital Cable, HD and subscriber channels offered by your cable company. If you would like to learn more visit Chris Lanier’s CableCard FAQ.