As general guide, only three parameters must be met: hardware needs to be quiet, sleek and reliable. Of that three, being quiet is probably the most important factor. After all, you'll place this thing in your living room where something as noisy as jet engine is not preferred. Noise reduction can be accomplished in several ways, most efficient being – reducing number of fans. Even before you start picking components for your system, you should eliminate all motherboards with fans on their northbridge chips and all graphic cards with active cooling. My system, for example, has only two fans: one on power supply and one on CPU. Fans should be large: 12 cm fans can provide decent cooling performance when spinning at less than 1.000 RPM. Of course, less RPM means less noise.
As CPU generates most heat in today systems, it requires strongest cooling mechanism, meaning it generates most noise. Choosing CPU with minimum power consumption will allow you to pick slower (thus quieter) CPU fans. One solution, a bit expensive though, is to pick CPU meant for laptop systems as they are optimized for low power consumption (less heat dissipation, slower required fan, less noise). Many manufacturers today offer desktop motherboards that supports mobile processors out of the box, while others provide additional adapters. Solution without adapter is preferred, of course. For my system, I choose Asus P4GPL-X motherboard coupled with Asus CT-479 adapter that gave me ability to use Centrino 2 Dothan mobile CPU. This approach have two drawbacks: adapter gave additional 5 mm in total CPU height meaning that I can't use stock coolers for Socket 478 (which I wanted, because cooler that comes with CT-479 is not as quiet as I preferred), and it also disabled software CPU frequency scaling that has major impact on CPU's power consumption (remember: no frequency scaling == more power consumption == more heat == louder fans required). Luckily, today are motherboards that supports mobile processors without any adapters. Check for Asus section in “mobile on desktop” category and for example their model N4L-VM DH that supports Core Duo processors natively.
And one more time - I can not stress enough how quietness is important. One complete PC configuration was wasted (costing me about 300 € in lost value after I sold it) before I learned how to build quiet HTPC. But don't get me wrong - my first generation HTPC was not loud, it was even quieter than average. Unfortunately that was not nearly quiet enough for living room environment where from time to time someone has to sleep, listen to quiet music or simply do some creative work. And neither of that is possible with 5 fans roaming at 5.000 RPMs inside your case. Simply, choose quiet hardware.
