First of all - technically graphic cards can work up to 115 degrees (celsius) yes, it is over magic 100 number.
Ask any manufacturer they will say - it is fine.
Problem - none of them have tested what will happen if you run your card for weeks at that temperature
And with any electronics cosmic law is - lower temperature = works faster, and will live for much longer.
We have hot summer too, but here is what I have:
gaming case with 3 coolers and proper airflow, HD6870 with 2 coolers on it
when I pushed mining settings to best I can it was running 85 degrees with coolers working 100%
.... then I opened case and cleaned everything, especially my card and it's two coolers
Now it runs 79 degrees with coolers on 60%
or < 60 degrees with coolers on 85%
Point is - if you want to make load on your card and pc constantly:
1) clean it often
2) get proper case with proper ventilation and coolers
my opinion - there is never too much coolers (I am adding 2 more high pressure intake coolers on my case soon)