Well, ya got one half of the Drake Equation going there. Check out:
http://en.wikipedia.org/wiki/Drake_equation
Yes, everything depends on the probabilities, but the more we find out about life on earth and how many really hostile places it seems to thrive in, its hard to think the probabilities are that low at all, even for "intelligent" life (dolphins are intelligent, man is not in my book
).
And as I always say to my kid, "do the math". Just using your own figures, you've computed 300 to 500 stars with intelligent life around them just in *our galaxy alone*. Innumerancy is an uncountably costly dilemma...
Cheers,
Buffy
