It was a German company called Hush Technologies, but they don't seem to be around anymore. That was way back before HTPC's had quite taken off, so there wasn't much market for silent PCs (if you could even say there's much of one now). It's well-made--the case itself is an impressive and heavy piece of solid aluminum industrial art.
Well, my tips are just by observing what this outfit did with my server, and I'm not sure I have a lot more I can add. A power-brick style power supply (similar to those used in laptops), just seems to make a lot of sense for this application, but I'm not sure where you'd get them and how compatible they'd be with various cases and motherboards.
Intel publishes the TDP of their processors on their website--I would not assume that Celerons run cooler just because they're less capable. The TDP numbers are actually a good way of comparing the different chips to each other (lower TDP is cooler/better), even if they aren't exactly real-world usable numbers in their own right.
I'd really want to caution you about the hard drives, though. If you're really serious about making this thing quiet (and if it was sitting next to my TV, I'd want that too), 3.5" drives, even the quiet 5400 RPM ones, turn out to be obnoxious once the system fans are gone and no longer cover up their noise. If SSD is out of the question, I'd really go for 2.5" drives, 4200 RPM if possible. I think Samsung Spinpoints are very quiet, although I'm not so keen on their long-term reliability (just speed up your normal replacement cycle). The system I've got has an "adapter" for placing 2.5" drives into a 3.5" spot, which is basically two metal plates with a squishy material in between them. You sandwich the drive between the plates, screw the plates into the chassis (with rubber grommits because you can never use enough squishy material), and then the drive vibrations are pretty well dampened, although it's still the noisiest part of the system.
That said, lots of people say they can't even hear the "quiet" PC fans that are out there, and you might be lucky enough to be among them. Determine exactly how quiet you can tolerate FIRST, because targetting absolute silence is a path to craziness.
EDIT: I also wouldn't worry about RAM, except that it will need to fit under whatever monster heatsink/cooling system you end up using for the CPU.