What if I buy a super-duper-computer with 2 or more top-grade nVidia/ATi GPU boards and will mostly (above 80% of time) use it for simple browsing/office/coding tasks, will it consume a lot of power and produce huge amounts of heat or will it just downclock itself automatically?
If you have modern graphical cards and you use them in CrossFire (AMD's name) or SLI (nVidia's name) then an unused secondary GPU can power almost all the way down. AMD calls it ZeroCore Power technology. I am not sure what the nVidia name is.
Note that this will only power down the currently unused card(s), not the main card.
The main card will keep using power. How much will vary between regular browsing on a single screen, regular usage on multiple screens (GDDR Speed increase in that case, raising power usage) or when gaming.
During low use/idle states most newer cards don't typically use much power. Normal use of your machine won't be taxing on your GPU.....so there would be little power consumption.
But, at the same time, regardless of the manufacturer...one card will of course use more power than another in any situation. Most high end graphics cards have power management systems (which both nvidia and ATI offer) that allow you to adjust voltage use by your GPU and so-on.
Hope this also helps: GPU Power Management