I have my E8400 @ 4ghz w/ 1.336v (CPU Z loaded reading).
With XIGMATEK HDT-S1283 for cooling.
HM max shows 65 C.
Real Temp max (which I mainly go by) shows 60 C.
Motherboard/system temps of 26-28 C.
Ambient ~69-72 F.
Still need to get it 24/7 solid, Orthos (small) stopped @ 1h..needs some more voltage.
Unfortunately, I've added about 6-8 C (aka a lot of core voltage) by going from a 3.8ghz clock to 4ghz...so I'm not sure this will be worth it for 24/7, other than knowing the chip WILL do 4ghz.