Like Unnamed25 said, the odds of forming a decent ring from a collision are poor, but the odds of forming a moon, I'd say, are pretty good: if the collision hits just right, you'll get a bunch of Earth Fragments or Impactor Fragments in orbit, and I'm pretty sure they become gravitating objects in their own right, so if everything works out, they could coalesce and form a moon.
Also, there's been talk that a future update to US2 will include the ability to run Smoothed Particle Hydrodynamics simulations, which are the same kind of simulations they use to simulate actual moon-forming collisions. Which would be awesome.