What would happen on Earth if a similar sized planet were headed on a collision course with Earth?

Just saw Melancholia, and this question came to my mind. My wife thinks chaos would happen on Earth long before the collision - atmosphere disruption, gravity forces, etc. would just kill everyone long before the planet strikes. It'd be illogical to be just standing there watching the bloody planet smack you right in the face.

So, what do you think? Love to know the answer, just out of pure scientific curiosity. Cheers all!