Fan speed regulator is definately the easiest one. If you have a high rpm fan sometimes buying an 80mm fan with low (2000-2500rpm) might be as cheap as getting a fan speed regulator.
What kind of heatsink and CPU do you have? In order to succeed with creating a quiet hsf solution you need to have a good heatsink to compensate for the loss in the active cooling the fan provides. This of course needs to put into context with which CPU you have since... faster cpu = more need for cooling.
/Matt