I don't know what kind of experience you have with overclocking, but I suggest you try giving my guide a read through which will hopefully cover most of your concerns.
To answer directly, 72C is the Tcase but you are probably ok up to 80C in burn tests, though it's unknown if that will decrease CPU longevity.
Is there a reason you need 5ghz, or just to say you did it? Because seriously that is really, really high and I would be shocked if you can do it on air.
I don't know what kind of experience you have with overclocking, but I suggest you try giving my guide a read through which will hopefully cover most of your concerns.
To answer directly, 72C is the Tcase but you are probably ok up to 80C in burn tests, though it's unknown if that will decrease CPU longevity.
Is there a reason you need 5ghz, or just to say you did it? Because seriously that is really, really high and I would be shocked if you can do it on air.