Personally, I think that computer you link is a serious ripoff. And when I say "serious", I mean SERIOUS. Those guys are selling you each component with a big plus in the price compared to the one at what they're sold in your normal computer store.
I suggest you to go DIY. A good resource for silent computers is Silent PC Review
. Those guys, despite being almost crazy almost silence, are quite trustable and their tests are very rigorous, and it's one of main sources of information despite my idea of silence being "everything that gets masked by some loud music"
. They also make their own systems, and this one
it's pretty good, both in loudness results (20 dB, which means you'll breath louder than the computer) and in power. If you can't order one (I don't know if they send products to Europe) you could use it as a rough guide for the idea of computer you have in mind.
Two things: if you can, throw that mobo away. Nowadays, almost any maker is selling passively-cooled motherboards, using heatpipes and massive combinations of fins, except DFI up to some extent, and that is changing. Second, DON'T install a fanless PSU, doing that is asking for trouble. Reputable makers don't have fanless PSUs, but instead they use thermally-controlled fans, which are virtually silent, and the fan only ramps up when you really need some juice and the PSU is being stressed, which, if the PSU is powerful enough, don't happen at all. With a fanless PSU, you're playing with fire, all the ones I saw are being built by dubious makers, they're inefficient (another point to have in mind if you want a good and silent PSU, as high efficiency means less heat the fans have to dissipate) and you need a really good airflow inside the case if you don't want them to go boom! at the first sign of serious work.