Q: Powerful mini ATX gaming PC and extreme low power hungry download server in ONE I want to build a really powerful gaming machine with some kind of a media server streaming full HD contents to the whole room and TV, but at the same time, a "magical button" which turns this power hungry machine into a really low power consuming web, download,.. server. 
I mean, is it generally possible with the present Hardware to switch to an extreme economical modes ? I am talking about CPU, graphic card etc.. The switch could be from 400 W to 30 W or somehing about these values. Of course, in the second low power mode, I do now want any performance. Try to turn off everything what is not needed for a basic network functionality and HDD. I would say, that the button should transform this pc into a very quiet, slow pc. Or, if possible, after a period of time, automatically switch into this mode.
I am asking because I was using an asus rt n16 router with DD WRT on it router for this low power purpose, but... 1 PC is 1 PC. And it wasn't really stable by streaming etc. 
Thank you

A: An idle CPU and GPU will consume less power than one fully loaded.  This has been true for a while.  Modern Intel and possibly AMD CPUs have technologies that allow changing of the clock rate by the operating system (read the Intel SpeedStep Wikipedia article), and this is supported by all modern operating systems.  
Hard disks can be powered down when not in use.  In the Windows power control panel, there's a plethora of power-saving options that you can experiment with.
The "extreme economical mode" on the PC platform is sleep, hybrid sleep, and hibernation modes.  There's a lot you can do to manage power usage on a PC (and desktops/laptops now consume less power than before, so you're getting more bang for your power nowadays than ever before).  But, there's no hardware you can add on to a PC to make it go into a super low-power mode.
You ought to offload your download tasks to an ARM based system such as your router or something like a Raspberry Pi, and shut down or hibernate/sleep your gaming system when not in use.
