I skimmed Stormin Mormon's post, and while I understand many people in the southern US view attic ventilation as something needed to cool a house (or upper floor of a house) in summer, but the truth is that an unbearably hot attic won't harm your house at all, and so in those cases attic ventilation is desirable, but not necessary.

Where you really NEED attic ventilation is in the winter when warm moist air rises up into your attic and forms frost on the cold roof rafters and roof sheathing up there. That frost doesn't do any harm......until spring when it melts and drips off onto the ceiling insulation. Fiberglass and cellulose insulation work by keeping air stagnant, so wet insulation takes forever to dry out, and wet insulation in contact with wood ceiling joists is a recipe for wood rot growing on those joists. And, as the cherry on the cake, the vapour barrier between the wet insulation and the ceiling plaster or drywall is going to prevent the ceiling from showing any signs that it's wet, thereby lulling the homeowner into a false sense of security that there's no serious problems up there until the joists are so rotted that the ceiling starts to sag under it's own weight and it's too late to avoid major costly repairs to the house. I myself have seen a picture of one house in Winnipeg that was used as a marijuana grow operation that had 3 inches of frost coating the roof rafters and sheathing in the attic.

If you live where it gets COLD during the winter, then read on...

If it were me, I'd go up to my attic today or this weekend and snoop around for any signs of condensation or frost on the roof rafters or underside of the roof sheething. If you don't see any frost, condensation, or mold on the underside of the roof, or dark, dirty looking areas on the top of your ceiling insulation, then you've already got enough ventilation already and adding a ridge vent is only going to waste some money.

(The dark spots on top of your ceiling insulation will have been caused by water dripping off the roof and depositing dust and dirt from the roof onto the ceiling insulation. Check under and around any dirty spots on the insulation for moisture, and if it feels damp there, check for wood rot of the ceiling joists.)

If you don't see any evidence of condensation or frost forming in the attic, I wouldn't mess with success. The evidence there is that you've already got enough attic ventilation so adding a ridge vent is just gonna waste some money.

(I'd put the ridge vent on the cathedral roof cuz you can't get in there to inspect it, so it's better to ventilate the he11 out of that part of the roof rather than risk insulation getting wet up there. Also, if you've had any problems with ice damming on your eves, you'd do well to add more attic ventilation to keep the roof cooler in winter.)

If you do see any evidence of moisture up there, then I'd leave all your existing holes in place, and add the ridge vent when they're doing the roof.

My thinking on this is that once the warm humid air from your house rises into your attic, it's not going to do you any good any more (by being warm and comfortable to be in), it can only do you harm.

So, since there's no benefit in retaining that escaped air, in my humble opinion, the more holes in your attic to allow that warm moist air to escape, the better.

But, I'd be concerned that without the power vents, if you didn't have enough ventilation, there'd be no way to increase the amount of ventilation, so that's the reason I'd keep the power vents. But, if you only had power vents, then you might have a problem at night with those power vents keeping you awake cuz of their noise and vibration, so my gameplan would be to allow for natural ventilation and keep the existing power vents so you have use them when and if needed.

Either way, if you live in the south where the heat in attic spaces can get unbearable, I'd want to keep the power vents to cool the attic in the summer if and when needed too.

Ridge vents, IMHO, are a bit of a scam. You should know that the only claim to fame ridge vents have is that they THEORETICALLY allow 100% sweep efficiency of the attic space because the unavoidable heat loss through your ceiling insulation drives a convective air current that draws cold dry air in the soffits and pushes warm moist air out the ridge; keeping the attic dry in the process. Or so they say.

THE PROBLEM IS that it only works that way on a perfectly calm day. 99 percent of the time, there's some wind outside, and cold dry air blows in the soffits on one side of the attic and pushes any warm moist air in the attic out the soffits on the other side of the house. That is, MOST of the time your soffits are ventiling the attic cuz of the wind through it.

Do this:
Take a cotton shoe lace, pop up to your attic, get one end of the shoelace burning and blow out the flame so that it smolders. Now use that smoldering shoe lace as a poor man's smoke pencil to see how the air is moving up there. If there's any wind outside, then that smoke will be blowing all over the place rather than rising calmly up to the ridge. So, if that smoke isn't rising up to the ridge then adding a ridge vent isn't gonna help very much at ventilating the attic.

(Some people argue that the wind blowing over the ridge of the roof sucks air out of the attic through the ridge vent, but a triangular roof is hardly the right shape to generate aerodynamic "lift" like a wing profile does, so how much good that does is probably negligible.)

Your contractors are gonna say: Don't keep the gable and power vents cuz that will screw up the theoretical 100 percent sweep efficiency you would supposedly get with soffit and ridge vents alone. But, the answer to that question is that you only get that 100% sweep efficiency on a few perfectly calm days every year. The rest of the time the wind outside is causing drafts in the attic that don't give that weak heat loss driven convective air current a chance to become established.

So, if it were me, I would check the attic for evidence of condensation, and only install a ridge vent if you find any, or have had problems with ice dams on your eves in the past.