I had overclocked my news core I7 2600K, I then ran it for about a day then while configuring bios found that my temp was 87 degrees Celsius! I then minimized the clock, and got it back down to about 43 degrees. Would that 1 day of high temperature operation cause the CPU to fail early or cause me any problems in the future, it is working fine now, i am just worried about it.
Nope it shouldn't of damaged it at all if it did you would know about it by now. While cpus are not meant to go above 72c or whatever it is they can and will last quite a long time at even 90c so i wouldn't worry, although wouldn't recommend 90c either ;p
The temps in the BIOS will always be higher than in Windows. The CPU cannot go into low-power idle mode in the BIOS, so temps will slowly increase. If you use the stock cooler, they will increase a lot, but if you have a good cooler then they won't increase so much while in the BIOS.
For a Sandy Bridge CPU, keep your CPU voltage below 1.4v and your core temps below 75ºC and you should be fine for an everyday overclock that lasts years.
I have personally verified the thermal behavior of Sandy Bridge CPUs. The TjMax (max safe core temp) is 98ºC, and the CPU will start auto-throttling (automatically lowering speed) in the low-90sºC to prevent damage.