DenGuru has an article up, and has been going on for quite some time, which is a DIY HD HTPC, Part I and Part II. They make it appear that making an HTPC is such a big deal, by going thru various chassis, power supply, motherboard, processor and memory. I don't know about them, I certainly don't have to go thru all the troubles of such. It's just so simple nowadays: go Intel® Viiv™ Technology.
All the critical parts are listed already, and just mention to your reseller that you're building a system based on Intel® Viiv™ Technology and chassis and power supply will be offered in so many designs and styles. Even noobies will have no problem choosing the components, and most of all, since it's Intel® Viiv™ Technology, you get full support from Chipzilla and OEM manufacturers.
So if you plan to build an HTPC, then just go ahead and make it an Intel® Viiv™. And yes, you can still make it a DIY, without making things complicated.
For more information:
Intel® Viiv™ Technology
Intel® Viiv™ Chassis Designs