I have only dealt with one factory built Home Theater PC (don't remember the brand). It was less than impressive. So on I went to build one myself. I am building my second one presently. The ONLY advantage to purchasing a ready made box is the fancy user interface that's included. If you know your way around the windows o.s. and other software, then there is no reason to buy an expensive, low performance, overpriced rig. Even if you do not feel comfortable putting the computer together yourself, you can have a local shop put one together for you. Instead of a tower case get a 4u rackmount case, all the other stuff will fit right in, just like a tower case. For the most part I agree with Crazy_mck's advice, except the choice of cpu. For my main computer I really don't care about the noise level. For the HTPC I think the lowest noise level possible is of paramount importance, therefore use a P4 cpu with a fanless cooler. The AMD cpu is an excellent choice, however it requires more aggressive cooling. I am using 2 5400 rpm drives at raid 0 to keep the speed up and the noise level down, I have a network backup daily so data loss is not a concern if a drive fails. Get a good sound card, the creative soundblaster AUDIGY is ok, but it wont do 24bit/96khz to record and playback. If you like high end stuff, look at the Terratec aureon 5.1. The video card should include a DVI connector, as this will soon be the standard, I think. The ATI 9700 All in Wonder is my choice. ATI is still hard at work with their drivers, but the hardware is there, soon the drivers will be up to par, I hope. If more is better, then Matrox offers some excellent solutions, especially if you intend to do serious video production.
Regards,
Genguy.
Documentation?!!