I found a better calculator (same people, but now with flash animation!):http://www.purdue.edu/impactearth/
For ****s and giggles:
An asteroid the size of the Empire State Building (381 m):
6.59 x 10e3 megatons. The average time interval between impacts of this size during the last 4 billion years is 1 x 10e5 years.
Crater is 4.5 miles across and 1760 feet deep.
If you were 50 miles from it (arbitrary number I came up with), you'd receive 2nd-degree burns. Trees, paper, and grass would catch on fire.
The earthquake arrives 16.1 seconds after impact at an intensity of 7.2. It's felt, but doesn't really do much damage to well-built structures.
The shockwave arrives in 4 minutes. Max wind velocity is 164 mph. Wood-frame buildings get knocked over, glass shatters, 90% of trees knocked over.
The internet is truly wonderful.