Generally speaking the narrower the gap the hotter the plug effectively becomes (the more intense heat between the electrode and ground).

Also in general terms the plug gap is determined by the ignition system used. When the car manufactures went electronic ignition in the early 70's with it's increased spark potential the plug gaps increased from about .035 to anywhere between .045 to .065. This combination also helped reduce emissions.

If your running a pretty much stock engine and ignition system, I would recommend starting with the manufactures sugested heat range and gap and then after driving for a while, pull the plugs and adjust the plug heat range accordingly.

One last thought, never assume the new plugs are correctly "factory gapped".