I wanted to share my concept of a Steambox that I made some time ago. The idea was to make a AV sized(43cm) wide case that would be able to support a decent low-profile CPU cooler, a high end GPU, and a boat-load of 2.5" HDDs. 3.5" HDDs would make the case too high, and they're louder and hotter. Personally, I feel that a HTPC case should be 43cm wide, because otherwise it won't fit in well with your everyday AV equipment. And seeing as a m-ITX system doesn't require more than 30cm or so even with a GPU; filling the rest with HDDs seemed as as good a solution as any. It'll allow for RAID-1 + RAID-5/6 setups for those who take storage seriously. Another idea I toyed with was integrating a SA-50 2x50w Amp board instead of half the HDDs.
Here's the concept:
The idea is to have each heat-generating component in its own section, with direct contact to both their own intake and exhaust vents. This way, as long as the components themselves have quiet fans, that'll hopefully be enough to keep them both cool and quiet without having to use case fans at all. The motherboard would be at the bottom left, the SFX PSU and slot-loading ODD stacked to the top left, the GPU is in the middle, and the HDDs to the right. There's a superfluous HDD above the motherboard, because I used pictures of the Valve Steambox concept for the internals. The tiny Zalman cooler isn't what I had in mind either, rather something decent in the low-profile, mini-ITX compatible category. Like the Thermalright APX
The CPU would be pulling air from the top, and exhausting it to the left. The PSU would pull air from the bottom, and exhaust left. The GPU would pull air from the bottom and exhaust out the back. And the HDDs would be lined vertically, with vents both to the right and up top, which hopefully would be enough to keep them cooled. The idea is that none of the heat-intensive components would interfere with each other.