Something like that can only happen to crappy PSU with unstable voltage output. Even with this, a motherboard is unlikely affected by it. The most sensitive part is the HDD, since I have a friend whose HDD died after his PSU blew up.
Then again, with a faulty low quality PSU, anything can happen.
More PSU power is always better in terms of reassurance from overload. But for low power requirement system, it would be like using high octane gasoline for a 100cc scooter. The rest of the power would come out useless.