Hypothetically:

Let's say I had a 15dbi antenna attached to the top of a tree, hooked up to 75ft of high quality coax, into my computer with ORiNOCO card.

Let's say my friend hooks up a 15dbi antenna to the top of his chimney, on a 2 story house. He hooks this up to a 1watt RF amp, which is hoked up to an access point.

How far away can we be from each other for this to work? The terrain is relativly flat, but there are lots of trees that might not be cleared by this setup.