the 4xx and 5xx have 3 sets of clocks in their bios 3d / 2d / idle
when you have 2 monitor they doesn't go into idle clocks, 2d sure they do
i've my memory is right on the 580(i don't have it right now) it'S 3d=777mhz 2d=450mhz idle=51mhz
temps tru the roof is bit exaggerated but of course you get muhc cooler card when using the idles clocks.
i'm running 3 monitor and nvidia doesn't allow that on one card so i brought a cheap 5450 for my second monitor and tv while the 580 was running the main display.
the "problem" is worst with the 4XX since they output more heat than their revisions the 5XX
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