ventilation is divided into 3 types. 1/ Those that vent by natural air movement, such as gable vents (When the wind blows, attic air moves too). 2/ Those that create their own drafting. This is done by having some vents high & some low (Like soffit vents & ridge vent), creating what many call a chimney effect (as attic air warms it rises & exits through the high vents, creating a negative pressure in the attic, causing air to be drawn in through the lower vents). 3/ Mechanical ventilation, like power vents & turbine vents.
For colder climate areas, we focus on the first 2 (natural & drafting), primarily focusing on creating a drafting effect. First determine the amount of ventilation needed. Many suggest 1 square foot of free air (Ventilation) per 300 square feet of ceiling surface area. Others recommend 1 to 150. Both of these figures are rather arbritrary. I suggest using the 1 to 300, as you can always increase later. Second, to create a drafting effect, 1/2 of the vents are installed high & 1/2 low in the attic. This can be done with any type of vents (Gable, cap, ridge, soffit). I've had good success using plain roof vents, 1/2 near peak & 1/2 about 2/3 down the roof, or just above insulation level. Or perhaps a combination or roof vents near peak & gable vents just above insulation level. The main thing is to have a seperation in elevation, the greater the seperation the better.
I suggest staying away from power vents, as these are for predominately hot climates & can have negetive effects elsewhere.