you are looking for the Drake equation.
http://en.wikipedia.org/wiki/Drake_equation
Every now and then someone has a go at updating our best guesses at each parameter. Problem is that it is not too hard to manipulate it to get the answer you want.
The real problem is that based on our current understanding of the laws of physics we could not have a meaningful conversation even if we did find ET let alone go visit each other.
Michael.