A major problem here in Texas is the heat in the summer. The sun beams down onto the roof of the house, heats up the attic, and that attic heat is passed down into the house. Insulation helps to reduce the transfer from the attic to the house, but lowering the temperature in the attic will also help.
We have vents in all the gables, but that only allows a small amount of heat to escape the attic. From what I read, the best scheme is to have soffit vents in the bottom of the roof, around the first floor of the house. This lets cooler air into the attic from the outside. This cooler air, as it heats up, will rise and then exit the roof thru a ridge vent that is put along the peaked ridge at the very top of the roof.
We contracted with Roofcrafters to install both the ridge vent (to let the hot air out), and the soffit vents (to let cooler air in). This costs $1200. Installing the ridge vent means taking off the roof shingles at the very top of the roof (along the ridge) and then using a circular power saw to cut the roof decking to provide an opening about 3 to 5 inches down from the peak of the roof, all along the ridge. Then a plastic ridge vent is put over this opening, and roof shingles reinstalled on top of the ridge vent. This prevents water from leaking thru the roof, but allows air out.
Then down at the soffits, a similar opening is cut all around the house, and a grill put over it -- to keep out bugs and such critters. Effectively this is just a screen, letting air into the attic.
Update: 23 June 2001. Now that we have a roof ridge vent, we don't need the turbine vent that was originally put into the roof. We had one of the wind driven ones, and then replaced that with a thermostatically controlled powered vent. We paid Drury Roofing $400 to remove the turbine, plug the hole and then roof over the top of it.