I'm running mine with a 5830 using Trixx to control them.
There is a custom fan setting (instead of auto) which I think most of the other overclocking tools have and you can set your fan to whatever you want. You could set it straight to 100% but that can be noisy.
When mine hits >85 degrees it chokes. Often with the auto setting the GPU heats up faster than the software seems to expect so it will overheat and choke before the fan can catch up. I get it reasonably cool using what the custom setting recommends and then upping it a little bit. The default setting puts the fan speed at 1%/degree so 75 degrees = 75% fan speed. If you change it to starting out high, like around 40% then hitting 100% at around 90 degrees then it seems to manage fairly well. I have it going from 74-80 depending on the outside temp.
My 6870 still runs much hotter than my 5830 and doesn't go that much quicker, but it's still a decent 2 card setup.
I'm also working on a solution to get rid of the heat that pours out of the fan port from the two cards and additional cooling for the 6870. I am pretty sure my case setup still has some tweaking to go.