I'm not entirely sure, but I believe as long as it's below 100*C, you're fine. The default threshold temp is 127*C before the core is set to slowdown by the driver, and I'm sure nVidia has a reason for it. However, if you're getting some artifacting or freeze-ups, then it may be time to check out the cooling.
I'd check the display properties in Windows, as it shows you what the temperature is thanks to a built-in temp probe.