An amazing computer visualization by the Discovery Channel (reported here by IFL Science) shows the damage caused by an impact of a 500-km asteroid with our fragile Earth. Basically, many things would fry. An ocean of magma (melted rock) would be created. Hot debris would rain down thousands of kilometers from the impact site. School would probably be cancelled.
Be sure to also check out the link to the impact calculator mentioned at the end of the IFL Science post. It tells you what would happen in the event of collisions with different parameters, such as impactor size, velocity, composition, etc.