I've got an Asus 8800 GTS TOP (also factory-OCed). The fan speed does increase when the card gets hot, but apparently, not enough. For example, Assassin's Creed manages to die rather quickly.

What I did was install nVidia's nTune and manually set the fan speed to 60% before starting the game (that's about the point before the fan starts getting louder exponentially). Ran the game at 60° C with no crashes.

I wish they had something like a target temperature or a fan speed gain I could tweak. It's rather inconvenient switching forth and back between manual fan control and automatic fan control each time I start the game.