To understand how hot each board can get, I measured the external temperature of each board's CPU while running nbench. Temperature was measured using an LM35D temperature sensor held against each board's CPU with tape. The temperature sensor was connected to an Arduino analog input, and a precision voltage reference was used with the Arduino analog reference input.
Note the Arduino Yun's CPU is covered by a metal RF shield. I chose not to remove this shield and measured temperature of the shield directly above the CPU.
From the results above you can see the Intel Galileo board runs quite hot, at around 60°C / 140°F. The CPU on the Galileo does not have a heatspreader or large package to absorb heat so higher temperatures like this are likely expected. According to Intel's datasheet
the Galileo processor can run up to 70°C, which is in line with the data that was measured here. In general be careful of touching or obstructing the CPU on the Galileo since it is very hot to the touch.
Another interesting result is the Beaglebone Black compared to the Raspberry Pi model B. Although the Beaglebone Black has better performance and power consumption, it appears to come with a trade off of noticeably higher temperatures (about 5°C / 10°F higher).
Most of the boards run at or below 40°C / 104°F which is not excessively higher than ambient temperature (about 22°C / 72°F at the time of these tests).