If you have a 15C idle temp in an air cooled system either:
It is winter time and your system is in an unheated room or
You have a malfunctioning thermal sensor.
Idle temps in an air cooled system (or most liquid cooled systems) will never be lower than ambient air temperature.
And 70C is at the upper range of acceptable for load temps in a Core2 system.
Like acer said, your components will make a big difference. This means everything:
motherboard, CPU, GPU, case, fans, construction choices including cable routing, room temperature.