I wouldn't worry about the temps so much as the noise from the stock cooler going at full pelt

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Anything up to 70 degrees should be fine. The problem is if you get dust build up and the temps increase without you realising then you want to have plenty of breathing room before hitting 80-85 degrees.
Whilst we're at it what temperature are you reading? The core temp or tcase one?
edit:
You don't want to be worrying about the individual core temps, just the tcase temp as that's what intels death temperature is measured against. You'll probably find your tcase temp is around the 45 degrees mark in which case you've got around another 40 degrees to be playing with.
Only bother upgrading the stock cooler if the noise is annoying you.