As Stated, Temps are OK. Gaming will not load the cpu as much as prime 95 and I would expect a drop of approx 5C for Games.
Max temp for the I5-2500k is around 95 C at which point the protection circuit kicks in and throttling takes place.
Try running furmark in a window and monitor temps for GPU and CPU. (also Not that the +12 does not drop below 11.6 V (My limit, spec is 11.4 V)
That said:
(1) stock HSF = NO overclocking.
(2) The cooler the better, I never even install the Intel HSF, for about 30 bucks you can get an excellent HSF, The Hyper 212. I have the Zalman 9900Max (TOO EXPENSIVE) and my I5-2500K @ 4.2 GHz and running prime 95 just hits 60C.