My research started at the power supply, and there are very few of those that have an efficiency of >85% and fewer still of those that are fanless (or have a small fan that only comes on when a certain temperature is reached).

Next was the mobo/chipset/cpu. This is where things get complicated…. as far as I can tell only Intel supply power usage information about their chipsets. So I had to do quite a bit of research to find an acceptable feature set for a low power draw.
I like the look of a gigabyte GA-MA78GM-S2H coupled with a AMD Athlon x2 5050e (low power, 45W TDP) as the gigabyte has a IGP capable of 1080p over hdmi. I was worried that all those built in features will suck juice even when the PC is idle. But I was relieved to find a tech report website that measured power used by the mobo and a 4850e at less than 50watts on idle, and just over 100 full load.
I wont be using this as my player for bluray as i have a great standalone, but i read that this board played high data rate blurays without stutter using a sempron.

Now I just need to get a couple of low power hard disks and set them up as raid-1 and I am almost all set (WD Green jobbies look like the ticket). I might even go one step further and get a SSD for the OS and apps so that the two hard disks can spin down, it would make the machine a lot more responsive as well. My thinking on that is that my family website + picture library etc would get hits often and getting them from a SSD would be instant whereas spinning up the WD disks to serve a page could be slow for the user. That leaves my raid for backup of family movies and snaps and other data that would be a pain in the backside to lose.

So, in closing, dont get quiet, get low power – its usually quiet by its very nature.

TiredGeek – plenty of good advice, thankyou! I’ve recently been impressed by the Domino ALC water cooling system, so may go for that at some point in the future. I found that a powerful CPU was held at 35 degrees even when running through our benchmarks, so there’s definite overclocking headroom too.

Peter – I heartily agree with your views on quietness. You wouldn’t accept a loud laptop, and wouldn’t accept a car that made dodgy noises, so I don’t see why PC users have to put up with juddering, rumbling machines when they unpack their brand-new computers. It’s a shame that quiet computing will only be achieved by those willing to spend extra cash and time tinkering, too.

An EC standard for quiet computing would be interesting, too – how would you see that working? More relaxed boundaries if your machine is more powerful? :p

Stock CPU fans are a real pain in the ear, so removing them should help, though a slow fan cooling your CPU is no good if the bloody thing rattles & viobrates when running slowly, as one of mine does….

There should be an EC “standard” for all manufactured PCs. If my car has to be “quiet” then I’m sure its not beyond the wit of man(&woman)kind to produce quiet PCs.

Whilst I could never condone paying the Mactax their all-in-ones are quiet, though they are also pretty gutless for anything requiring serious graphics or computational grunt (i.e. games). The key to cooling is passing a decent volume of air over the heat-exchanger. Big fans can shift larger volumes per rev than small ones but this is no use in a micro HTPC enclosure, but a good general rule when selecting a case.

Watercool everything if money is no object

Silverstone tried this recently with their Raven RV01, but sadly they seemed to have botched it with an over-priced, ugly and poorly detailed implementation. But this does show it is possible and maybe we can hope for a better implementation in future?