The limiting factor to overclocking the I7 4790k are rather voltages than temperatures, if having a potent cooling solution. 1.3v is about the maximum I'd recommend for everyday use, 1.4v for testing on high end air (noctua nh-d14/15), high end AiO (h100i, 280l) or better cooling.
For 24/7, you can expect about a 4.6-4.8ghz overclock.
However, back to your question. If you can pick the I7 4790k for $279, by all means go for it. That deal is excellent and you won't find a better cpu for that price.