This seems like a simple thing, but it's actually pivotal, for two reasons.
Atheists do not believe in any gods, not just one specific "God". You surely wouldn't say you don't believe in Harry the Bigfoot, you would say you don't believe in any bigfeet at all. Same with gods. When you capitalize the singular word "God", you are talking about somebody's specific god. Why do that?
This is why the issue is so important. When you tell a theist you don't believe in God, his God, you make it personal. If religious people are insulted when you say, "I don't believe in God" maybe it's because you insulted their particular god and not gods in general. Why go out of your way to irritate this religious person? What does that get you? Respectfully tell him you don't believe in gods. Then you are not picking on him and his faith. He might not like it, but at least you are not insulting him out personally.
So when you bring up that you are an atheist, it's better to simply say you don't believe in gods. I can tell you from personal experience, if you're not a jerk about it, most people will let it slide.
(And as a footnote: If it's your goal to piss off religious people, why? Most of us want to reduce the influence of superstition in our society, and the way to do that is to gain widespread respect for our skeptical ideas. You get respect by being respectful. Fighting with some dude on the bus doesn't help us in any way.)