I like to keep mine (ATi 5870) under 60C if at all possible, this means a 45-50% fan speed under full load (folding/gaming/both at once). ATi's 5xxx cards consume less power than the GTX4xx equivalents so they won't need as much fan power to cool them. nVidia's new cards do run a bit hotter so you may not be able to stay under 60 without really pushing the fan (and the noise). I'd stay under 70C either way as after that you can affect other things.
While the chip may be rated for 90-100C, the heat can damage the solder connections between the chip and the graphics card PCB. This is the most common failure in most modern electronics related to heat as the solder can fail at lower temperatures than the chip can. The solder failure is accumulated, it doesn't happen from just overheating alone, it happens after many heating/cooling cycles wears the solder joints out (this is because metal expands/contracts a relatively large amount from temperature changes). This problem was the cause of the Xbox 360's infamous Red Ring of Death problems as well as most laptop graphics failures as both cases involve inadequately cooled GPU's.
