I’ve done some naive overclocking of my watercooled 840 at 3.6Ghz at 40, which is about the temperature it was running before overclocking– This is run UD agent as the load.
This article says they overclocked the 840 at 3.5Ghz at 70 degrees, but says water cooling can get you to 3.7 or 3.9Ghz
This article reports instability at 77 degrees at 4.0Ghz. They said 3.8Ghz runs fine with an air cooler.
This article reports stable at 4.04Ghz with a water cooler and 3.8 without a water cooler.
This article reports stable at 4.0Ghz with a quality air cooler, and instablity at 4.2Ghz.
This article reports stable at 3.7/3.8 air/water and instable at 4.4 water cooled.
Here is an opinion that the maximum temperature for 840 is about 70 degrees
Intel says the 840 should be at most 69.8 degrees.