Seems like I was wrong. Top card has a harder life.
Guess I'll switch the position of my 5870 and 5850 then, since the 5870 was on top and is running at 86 Celsius when mining (at 96% fan!) while the 5850 underneath enjoys 70 degree temps at 52% fan.
It's always about airflow. Which is why middle cards typically tend to be hotter when you go above 2 cards. They have a hot card on top of them with poor airflow radiating heat and disallowing it to radiate efficiently, as well as additionally having it's own airflow reduced by a card below it (also radiating heat into the area it takes it's air from).
If you can figure a way to space the cards a little bit (risers, or plastic spacers) and/or force cooler air in between the card it can help a LOT. From my own experience when the room ambient was about 80F the cards ran hotter and louder than when the room ambient was 65F, without any other change to the setup. Cooling is a PITA.
Ambient temperatures make a huge difference. The lower the ambient the more energy it can absorb. a 20 F change is not insignificant. I have my fans all at 85% and they only reach 60c, because i make sure my ambient stay from 65-70f. But i have to agree with the middle being hotter. But proper airflow will fix that. Just make sure that there is enough positive air pressure to allow the fan to suck that air in, I prefer blower style cards for better cooling, but with the open style, i just like to have a strong exhaust fan. I like to keep things neutral, I usually do 2x65cfm intake fans right behind the cards and one 110-120cfm exhaust at the top. Seems to keep things manageable.
