1) If you can overclock by 10%, then you get 10% better performance. How you use that extra 10% is up to you.
2) The CPU will melt at 90 degrees. It is not uncommon for even an un-overclocked CPU to exceed 60 degrees under load. Once you start overclocking the thermal load increases significantly, making 90 degrees a possibility.
Also, the cooler you can keep your CPU, the higher you can overclock it. CPU's (as well as almost all semi-conductors) perform better at lower temperatures. I'm not going to go into great details why, just accept that a CPU kept at zero degrees is more efficient then the same CPU kept at 60 degrees. Because the CPU is more efficient at lower temperatures, the better you can cool it the higher you can overclock.
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