To achieve the goal stated in the title, I want to combine a Haswell CPU (with a 0.05A semi-passive PSU) with its integrated GPU, a quiet CPU cooler for them and a SSD, all in a noise isolated case, for office & internet use.

For gaming I want the computer to have 4 cores and a graphics card like the 7870 or the GTX 660. From what I've read here and on the bumblebee project, the nVidia GPU turns on/off automatically under Windows 7/8 and needs only a modified command under Linux for those programs that need its power; when it isn't needed, it draws zero power, is that correct? Can I achieve that with an AMD GPU under Linux or Windows 7 (I've a key left)? Or do I have to buy a nVidia card?

And if there's anything else I should know when building a pc like this, feel free to share. It'll be my first self-built computer, and I'll use the "ideal gaming pc faq" from the German as a guideline, picking quiet components where possible. About my expertise: I am only a happy Linux user, but looks like bumblebee has matured enough (or is it me?) to be usable (with playonlinux) without much trouble :-)