A nice article by Michael Richmond <mwrsps@rit.edu> may be found at
<URL:http://a188-L009.rit.edu/richmond/answers/snrisks.txt>.  His
conclusion is:

"I suspect that a type II explosion must be within a few parsecs of
the Earth, certainly less than 10 pc, to pose a danger to life on
Earth.  I suspect that a type Ia explosion, due to the larger amount
of high-energy radiation, could be several times farther away.  My
guess is that the X-ray and gamma-ray radiation are the most important
at large distances."