The other device in your computer that will contribute to video quality is the TV tuner. Under Windows Media Center it is responsible for both tuning into a channel properly and recording it. MCE records TV in a special variation on the MPEG2 codec (similar to how DVDs are encoded), and because this is a rather demanding task there are special chips built onto the tuner card itself that handle this (usually complimented by a small amount of memory). The quality of the tuner itself and the MPEG2 encoder will determine how nice the TV signal will look when you are viewing it. ATI makes a great TV tuner chip, the Theater 550, which several manufacturers have created cards based off of. You will also need a program that can do MPEG2 decoding (like a DVD application) so that MCE can properly play back TV (and DVD). The software you choose for that role may also impact video quality, so I suggest getting something that takes advantage of your video card's MPEG2 decoding capabilities (all modern video cards have this feature - Avivo for ATI, and PureVideo for nVidia). There are also dual tuner models that have a pair of physical tuners (and encoders), allowing you to record and/or watch two different channels at the same time (like this one). The same thing can be accomplished using two separate tuner cards, but then a signal splitter would also be required.

Lastly, we come to what should probably have been the first decision: the case. Most home theater owners will want their computer to blend in with the other parts of their home theater setup, like receivers and amplifiers. For these situations, a nice, sleek looking, rack-style case is probably best. Silverstone makes several cases that work great in such a setup, including the LC17 and LC18. Other users may want a traditional mid-tower case, but one that is as quiet as possible. The Antec P180 is the perfect solution - it combines a larger, more upgradeable tower design with plenty of airflow and features designed to specifically quiet the noisier components of a computer (the hard drives and fans). Lastly, there may be people who want simply the smallest computer possible - perhaps for a dorm or bedroom. Again Silverstone comes through with the MicroATX-size SG01. It is also extremely portable, and has an optional duffle-bag-style carrying case.

There are also modifications that can be done to a case, such as replacing stock fans with quieter models and lining the inside with sound-absorbing foam. These sorts of add-ons will cost a little extra, but can make a noticeable difference if you want to make an HTPC that is as quiet as possible.