I know a lot of people consider agnostic a position on the spectrum of belief, and therefore believe it to be somewhat mutually exclusive to atheism. However everything I've ever read about it insists it is a knowledge position. And knowledge and beliefs are two different things.
You yourself write:
The Christian says: "I believe in God."
The Atheist says: "I don't believe in God."
The Agnostic says: "I don't know if there is a God or not."
The christian and the atheist are talking about what they believe, but the agnostic is talking about what he knows (or doesn't, or can't).
You can easily add to this and say:
The Christian says: "I believe in God, but I don't know if there is a God or not."
The Atheist says: "I don't believe in God, but I don't know if there is a God or not."
The Agnostic says: "I don't know if there is a God or not."
As to why I call myself atheist, as oppose to agnostic, well, this is the best metaphor I've heard for explaining it:
There is gumball machine. The number of gumballs in there can only be either odd or even. A woman looking at the machine says, "I can't know whether the amount of balls is odd or even." So she's agnostic. But then a man comes along and says, "The number of gumballs is even." And the woman asks, "How do you know that?" To which he replies, "I have been told, and I have faith that's it's true."
If the woman doesn't believe the man, (even if she wants to, but is unable,) then she's an atheist.
It's very important to note: just because she doesn't believe the man's claim that the number of gumballs is even, that doesn't mean she then believes the number is odd, yet she is still an atheist.