The current solution to preventing mold in attics is very low tech: vents along the soffits and a ridge vent. Here's what soffit vents look like: Images of Soffit Vents.
As you can see, there are many different kinds. The current rule of thumb is to have one square foot of vent space for every 150 square feet of attic space. And this means painted over vents probably don't hold muster. FYI: quotes on adding continuous vents on average sized attics run about $1300.
Here's what ridge vents look like: GAF Ridge Products
The idea is that warm air rises and then draws air up along the inside sheathing (plywood) in the summer AND winter. To most people's amazement mold forms in attics in the winter. Warm moist air from your house rises into the attic and condenses on the cold sheathing. I even have lots of mold forming on the ceiling of my carport every December due to the slightly warmer attic above encouraging condensate on the ceiling below. So adding a thermostatically controlled fan to your attic won't help prevent mold, it simply takes warm air out of your attic in the summer and can actually draw conditioned air up from your house too!.
Anyhow, the best way to prevent mold in the attic is to have continuous soffit vents and ridge vents. Gable vents and soffit vents usually work OK too. But the soffit vents are crucial AND THEY MUST BE KEPT CLEAR OF INSULATION. That's the job of the baffles shown above. Click on image above for link.
I also recommend foam boxes or stair tents over the ceiling openings and a humidistat switch on the upper floor bathroom fan to help prevent humidity reaching your attic. See Ventilation tag for these products.