Basically just the CPU fan, case, and PSU (if you're lucky). If someone can figure out how to get Doom running on just those 3 things, young people will be able to ascend.
Pretty much. In a typical motherboard, the fan controller is linked to temperature monitoring and the duty cycle is changed in correlation with another variable, generally CPU temperature. The PWM control method actually doesn't require the monitoring of fan speed at all, but it's included for informational and safety purposes.