It depends on a lot of things. If the car is stable at high speeds, there's no need to add the weight and wind resistance of a wing. If, however, your car is like mine and is all over the road at 130, things need to be done. Improving under chassis aerodynamics is typically the best way (lowering it and adding an air dam, like you said). Special hoods, like Griggs heat extractor hood also help by pulling air from under the car (resulting in low pressure) and allowing it to flow over the car (resulting in relatively higher pressure).
High wind velocity actually creates lower pressure (the same reason a plane flies) so when high speeds are reached, low pressure above the car actually sucks it up, resulting in less traction. Definately a bad thing cooking into a corner at 120. If the low pressure below the car is unable to compensate for the low (but relatively high) pressure over the car. Other methods must be employed. Hence, a wing to help provide additional downforce.