I've been thinking a lot about this topic, and a lot of the issues for me come down to the following:
1) RF noise: How much RF is your device going to throw off, and how much is it going to mess up the other components in the HT system? Correct me if I'm wrong, but the FCC standards for a computer aren't as strict as those for a stereo component, so I would expect that something like a de100c would throw out less RF than your run of the mill PC.
2) Audio noise: How much computer induced noise ends up in the audio coming out of the box? For instance, on my old PC at home, I can hear the SCSI bus through the computer's speakers. I obviously wouldn't have a SCSI bus on an HTPC, but this is still an issue, obviously. As a partial correction, I'd go for an optical link between an HTPC and the receiver if possible, as it would offer electrical isolation from the HTPC, as well as giving the resultant analog signal all the quality that the receiver (rather than the cheesy sound card in the PC) has to offer.
3) Accoustic noise: How much fan, hard drive noise, etc, are you getting?
4) Ease of use: How well can you put a front end on the HTPC thing so you can use a remote (preferably without having to watch a monitor), rather than using keyboard/mouse/screen?
5) How well does it integrate with the rest of the system? I wouldn't want to put a generic beige tower in the middle of my HT system, although truth be told, I'd like to hide the HT system in its entirety (with the exception of the screen).
To answer your questions:
1) The Audiotron, in my mind, offers several benefits. It gets all the noise (RF, audio and accoustic) out of the room that the HT system is in. It offers an optical out, meaning that you're not going to have to worry about a noisy audio signal coming from its processor. And it has a nice remote control. The downsides are limited display readability (from a distance) and the fact that it, like its competition, may not do so well with really huge libraries.
2) Find a PC case that'd look good in the HT system. People are actually making these. Investigate special cooling components that'd either make a quiet fan or eliminate the fans. For instance, look into lower powered CPUs that could take passive cooling, lower powered hard drives, etc. The CPU and I/O demands of an HTPC aren't that great, especially if used mainly for audio.
Most importantly, look for a sound card that's either A) High end or B) offers a digital output. Creative Labs may make a decent sound card, but it's definitely not "stereo system" quality unless you use the digital outs and let the receiver do the D/A conversion.
You know, other than that, I figure making a PeeCee based MP3 player has some considerable potential.