wrong. theres lots of other stuff involved like voltages. Too low and you get instability, to high and you kill your CPU. RAM timings change when you over-clock your CPU. etc. There are guides on how to overclock, that tell you step by step how to sort it out. After market cooling is also essential for larger overclocks.
Yeah, there are a lot of variables... Generally though... he's right. Especially if he has good air cooling.
Voltage is a huge factor like you guys are saying, but obviously the higher the voltage, the higher the temps. You can pretty much overclock until you're comfortable with your MAX temperatures and then stop. If you try an un-safe voltage, chances are your temps will also be unsafe. Just don't treat temp as the only variable you need to watch.
If you stay below 75C or so for a safe overclock, there's no way your voltages are that high in the first place unless you are using a good water setup or something... a 950 wouldn't need much more than 1.25V to reach 4.0GHz and that is no where NEAR dangerous territory.
Basically what I'm saying is that you can't have dangerous voltages without ALSO having dangerous temps. Just do your research on safe voltages and you shouldn't have anything to worry about except temps and stability.
What kind of a cooler do you have houdinii? If it's still stock, everyone here will say you need an aftermarket cooler. Nothing too fancy, but it's well worth it.
To answer your OP though, 75C is usually agreed on as a safe limit. 80C is not uncommon, but not favorable. Of course, I'm talking about temps at MAX load... with programs like Prime95, OCCT, IBT, or other stress tests. Gaming with never reach these temps unless your cooler is very very poor.
I haven't seen too many 950's being overclocked, simply because they're more expensive than 920/930's, but they are a higher quality binned product. 3.8GHz should be a cake walk.