I'm not sure what you mean by "conventional", certainly a plain resistive heater drawing 500W emits the same amount of energy as a computer drawing 500W. If you stretch the definition of efficiency, an infrared heater produces a similar subjective improvement to a room's temperature for less energy, as the infrared light skips heating the surrounding air and warms the subject directly. That is one downside to using computers as heaters - they produce almost no infrared, which can be less efficient for a given subjective experience.