I am running an Intel i7-920 chip on stock cooling and i'm getting about 55C on idle and 65-70C during light browsing (youtube, etc.) I understand that the maximum temperature the chip can take is 100C but I was wondering whether it will harm or damage the chip in the long run if it runs in temps such as the above. Also, would there be any kind of loss in performance if say for example it runs at 70C compared to for example 50C?
Temperature: If you are in a 20C office then 55C at idle is rather high.
As reference: My desktop has a I7-920, a GPU which disposed of its heat outside the case, 20 Watt RAID card and four hard drives. Using stock cooler on the CPU and 3 10cm fans the CPU idled around 45C in a 20C room. If your room temperature is higher then CPU cooling gets less efficient and your CPU will be hotter.
Replacing the stock cooler with a Nehalems (a huge after market cooler) lowered idle temperatures to 42C. At full load it made about 15C difference.
Ok, now I got the introduction out of the way. Onward to the question:
Q1: Yes, heat will eventually harm the chip. Please do not use it for decades with poor cooling. But if you want to replace it without 5 to 10 years then is probably is not a problem.
Q2 Performance should not be lost due to the temperature, but i920's use a feature called turbo boost. That allows one core to 'auto overclock' by one bin (which is 133MHz at default settings) if the other cores are idle and it has room to dissipate extra heat. (Running the same chip at a higher speed generates more heat).
This feature might run less long if your cooling in inadequate, meaning that there could be performance loss if you leave your system mostly idle with occasional bursty non-treaded applications.
There won't be any performance loss, but you could be possibly shortening the lifespan of the CPU by running it at hotter temperatures. That being said, I ran a 920 at 60C-90C for a year and it held up fine (overclocked + incorrectly configured watercooling).
Those temperatures seem a bit high for stock cooling and stock speeds. Is your processor overclocked?