Approaching a person
as an atheist is probably not the best idea.
It's important to remember that most self-professed atheists know at least a little about what they're rejecting. Some atheists know more about the technical aspects of Christianity than its practitioners. Giving them another presentation of the same ideas they've heard repeatedly will probably only more deeply reinforce reflex reactions.
In other words, an atheist is never more an atheist than when she's being preached to (or worse, preached at). By approaching a person as an atheist, you've jumped into those those deep grooves of habit and will have trouble getting out of them. You'll both be reading from scripts. Don't approach with a script!
I agree with the advice above - go with the Spirit. Otherwise, if you have something planned, you're going in under your own power and wit which, with apologies, will end in frustration unless God decides to use you in spite of yourself.
Now, concerning the rest, the practical atheists who profess a belief in God yet simply don't believe...