I can only speak for myself and my GTX 670, but I've been told and also found a max safe temperature s for most gpus. They've stated a max safe temp of 90ish, but you'd want to stay below 70C to 75C during gaming to prevent bottlenecking.
I would recommend downloading a gpu monitoring app. I use EVGAs Presicion X, (theres also MSI Afterburner that I know is another favorite) and I set a custom fan curve to ramp up the RPMs to 80% when the temps hit 70C. It keeps the card cool.