Most cards are good up to 99c but you should not let your temps go beyond 80c. 99c is burn out. Anything over 80c, you start to have issues like laging and game frezzes or crash to desktop.
Like DM186 has said, 80C is a good temperature, and anything above 90C is generally too hot. 70C is really where the sweet spot is at, but 80C is alright too. 90C will have you ending up with lagging and crashing and BSODing.