Wall matters some, trapping the heat. 70 is fine, tho, have you tried you fans lower? At 100%, they may not last as long as youd like them to. Theyll last, but will shorten their lifespan.
If you can stay in the 70s, youll be fine. 80s is touchy, and 90s are a no no, tho, some do run that hot, and theyre fine as well. Try turning your fan back to auto, game and check temps along the way
i have an ati3870 and i normally hit the same levels of temp. while playing heavy games such as crysis but when i config. the fan at 50% speed via Rivatuner i dont go over 70 degrees and the noise isn't very loud in comparison with 100% where the fan sounds like a plane....
under load, 70c is cool for a higher end Nvidia card. You have no worries
if the card breaks 90c then you can look into better airflow for your PC...or in michaelmk86 case not play crysis in the middle of the jungle for a more realistic feel =P
Maxxing any component, or part of it will cause undue wear on it, and ultimately shorten its life span. Over clockers sometimes find their highest overclock, but no one uses it 24/7.
The noise generated from the fan, the threatened lifetime reduction of maxxing it out simply isnt worth it, as everyone has said, your temps are fine
Αnd remeber that airflow,case quality,gpu fan quality,fan's rpm and the amount of dust in your case are ala variables that when mixed cause the temperature you see,so try cleaning your case and your gpu,remove all the dust and do stuf like that so that you can have the lowest possible amount of temperature,but generaly you have completely normal levels of heat so dont worry,be happy xD