5 GHz at those temps is a tall order, you'll need some serious cooling. My 2500k runs at around 74C at 5 GHz with a Hyper 212+ and a pair of Cougars on it. Single cougar cools just as well, but a pair can do the same job with less noise.
Really, the important thing to consider with temps is your voltage. Voltage has a much greater impact on temperature than clock speed. Keeping your temps low is all about finding the minimum voltage to get your OC.
That said, if you only want bragging rights on your OC, it's fine to run the 2500k in the 80s for a few minutes. If you ever break 85, though, immediately stop what you're doing and let the chip cool down. I've been running mine at 4.9 GHz because I can keep it under 70C and right at 1.4v to do it.
Takes more than 1.5v to get to my max OC of 5.25 (you can see the link in my sig), and Prime95 took it to 90C in no time flat. I can get to 5.35 if I turn off a pair of cores, running at a frosty 65C or so. Planning to try to hit 5.4 GHz as dual core this weekend for some single threaded benching, I think I can bump up my BCLK and hit it. It's been responding well to up to 104.5 BCLK so far.