I really enjoy talking about communism with atheists: even if they disagree (and most of them do), they listen to and engage with the actual arguments, and offer real, substantive arguments against communism. It's much more interesting and engaging than talking about atheism with theists, who just think differently (and stupidly) about knowledge and truth.
One of the interesting points raised in discussions with one of my atheist friends last night was the difference between theory and practice. There is capitalist theory, proposed by writers such as Adam Smith and David Ricardo; this theoretical capitalism is much closer to liberal democracy and democratic socialism than actually practiced in the United States and most of pre-Marx Western Europe. Indeed theoretical capitalism and liberal democracy differ sufficiently from practice that it's arguable to say they've never been tried, and it's arguable that European democratic socialism owes as much or more to Marx than to Enlightenment liberalism.
Likewise there is communist theory, proposed by Marx, Engels, Lenin and Mao (before the Russian and Chinese revolutions) and others; this theoretical communism differs in important ways from how communism was actually practiced in post-revolutionary Russia and China.
Many communists, myself included, say the most interesting argument for communism is to compare theoretical communism to practical capitalism. The validity of this comparison is controversial, and deserves argument.
Capitalism has been implemented in practice under more varying and favorable circumstances than communism. We have a lot of evidence of how capitalism works out in practice under a lot of different cultural and material circumstances, from Germany to Japan, from wartime to relative peace, from agrarian-dominated to industrial to post-industrial societies. After the capitalists (justly) defeated the feudal aristocracy by revolution, it lacked strong opposition. Capitalism also had two continents (North & South America) to grow without strong opposition.
We have, on the other hand, only two examples (the USSR and PRC) of anything remotely resembling communism and communist-socialism in practice, both desperately poor and both facing enormous opposition and hostility from the capitalist West (and, sadly, each other): the United States alone spent trillions of dollars prosecuting the Cold War, and if you believe this hostility was due to the authoritarianism of the USSR and PRC then I have a bridge you might be interested in buying.
Moreover it's a No True Scotsman fallacy to say that the USSR and PRC were simply "not communist". They called themselves communist, and they implemented many of the fundamental principles that distinguish communism from capitalism. But communism isn't scripture or dogma; it doesn't take anything away from either communism or the Russian and Chinese people to say that the communist leadership in both societies made mistakes, some of them serious and with catastrophic consequences. Furthermore, modern communists — like anyone else — can independently identify the mistakes by observation. The failure to comply with theory is best proffered as a causal explanation to account for the mistakes.
Capitalist theoreticians (other than pure lassez-faire Randian capitalists) make a fundamental mistake: they do not give a fully material, scientific account of political theory. There are elements of theoretical capitalism, especially regarding political control individual excesses, that we should somehow implement because they're the "right thing to do" in principle, without an account of the material foundation underlying how these principles will find expression in practice. And these principles do in fact lack a material foundation, hence they are honored more often in the breach than the observance.
Theoretical communism, on the other hand, insists that we identify a material foundation for all political theory, a foundation in the actual material, economic interests of actual human beings. Even if Marx were wrong about everything else, he deserves credit as a genius for this insistence on a fully materialist analysis of politics and economics, as well as his actual material analyses of economics and political-economic history.
Most importantly, capitalism is the status quo and communism and communist-socialism is the proposed innovation. We cannot ever justify an innovation on its own empirically proven practical effects; we can justify innovation only on theoretical grounds. We can, on the other hand, judge the status quo on its practical effects, not on its underlying theory.
No theoretician ever gets it right on the first try. Capitalism required a century or more to figure a lot of things out, and adopted even minimal humanistic and democratic reforms only under severe pressure from communists and socialists in the late 19th and early 20th century. (It interesting to note that it is precisely these minimal humanistic and democratic reforms that the capitalist "fundamentalists", including Rand, condemn in the strongest terms.) The Russian and Chinese experiments in communism and communist-socialism had many good results, and many bad results, but no one advocates that we do precisely as they did. Communists advocate that we do what they did right, and avoid doing what they did wrong. (Of course, figuring out which is which is decidedly non-trivial.)