The cards don't throttle down till 100C or so, 84C is not at all unusual for an 8800GT at very high load.
As for reducing it's life, that's relative. A GPU at 84C won't last as long as one at 70C, and one at 40C won't last as long as one at 0C.
Even at 84C, the card will probably last years.
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson