A PSU will deliver to your mobo several power sources with voltages regulated to exactly what the mobo requires - and these are standard for all common mobos. The mobo will "consume" from those whatever current its components need, and no more. the more-powerful PSU cannot force more power (current) into the mobo than it requires. Even if you mount an 800W PSU in a machine that is using 250W, the actual power consumed by the mobo will stay at 250W, and the PSU will be resting comfortably, hardly working at all.
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.