Here's the nominal operating range for Core temperature:
Core temperatures above 85°C are not recommended.
Core temperatures below 80°C are ideal.
Core temperatures increase and decrease with Ambient temperature.
Idle temperatures below 25°C are generally due to Ambient temperatures below 22°C.
Highest Core temperatures occur during stress tests or heavy rendering and transcoding, but are lower during less processor intensive applications. Gaming generally averages around 55°C, yet can range from 40°C to 70°C or more, depending on how a particular gaming title allocates CPU / GPU workloads, as well as differences in cooling performance and Ambient temperature.
Here’s a list of variables that affect Core temperature:
Ambient temperature
CPU Cooler
Fan / Pump speed
Thermal Interface Material
IHS / Cooler flatness
Core count
Core speed
Core voltage
BIOS updates
Turbo Boost
Hyperthreading
Instruction Sets
Memory
Case design
Cable management
Computer location
Ventilation
GPU cooler type
SLI / CrossFire
Dust