Hussein,
I have had many interesting discussions with atheists about morality, and from what I have learned from them (and read in books) it seems that ANY system of morality that is not derived from God is a system of morality that results in irrationality, and inconsistency. This is definitely something that I learned from reading books on presuppositional apologetics (especially Van Til).
For example, every time that I talk to an atheist about morality, I always look for key words such as: happiness, good, evil, bad, pleasure, pain, hurt, harm. ALL of these terms are relative terms, and their exact definitions change according to who you are talking to. Aristotle spoke of happiness as being the Ultimate Good. Hume considered things to be good if they were useful ('useful' being yet another relative term). John Stuart Mill advocated Utilitarianism, which asserts that we should seek out the greatest happiness for the greatest number of people (and I would add, for the greatest amount of time).
In all of these cases, a non-Christian philosopher is attempting to define a system or morality. The problem is that they end up just using relative/ambiguous terms to define other relative/ambiguous terms. The cycle never ends.
When you press them on these issues, they will eventually retreat to terms such as 'common sense' or 'common good'. These terms also need to be defined, and it ultimately results in the atheist declaring that morality is determined by majority vote (society decides what is right and wrong). Of course, this is an Ad Populum argument, and is illogical, since we would all agree (and every atheist I have spoken to agrees) that the majority does not always do what is right.
Furthermore, you have to ask them which society they adhere to. There are hundreds of societies in the world today, so how do we know which society is right and which is wrong? When we look at history, how can we say that the Nazis were wrong? The Nazis didn't think that they were evil. Simply because the Western Democracies declared the Nazis to be evil does not mean that they were.
An atheist will immediately respond that the Nazis were evil because they were murderers, killing millions of people. Yet the atheist does not realize that he is still trapped in his own circle of relativism. The Nazis themselves did not consider killing Jews to be murder (since they viewed Jews as subhuman, and enemies of Western Civilization). In Hitler's eyes, going to war against the Jews was self-defense. So who gets to decide whether Hitler was right or wrong? The atheist inadvertantly uses a specific human society as his final authority on matters of morality. Yet even in this he is inconsistent, because as soon as that society deviates from what HE considers to be right, he will disregard and condemn that society for being immoral.
In the end, the atheist cannot avoid using relative terminology to redefine other relative terms. As for the so-called 'final authority' on matters of morality, the atheist often uses a specific human society as the majority vote (ie. 21st Century America). Yet the atheist is not consistent, in that if society does not agree with him, he will criticize society. In this way the atheist reveals that HE himself is his own final authority on morality (he makes himself his own god). That is why any system of morality that does not derive itself from God is always going to lead to moral relativism, irrationality, and inconsistency. In order to defend the existence of objective morality, the atheist MUST borrow from a theistic worldview. All we as Christians have to do is catch them in the act.
I would love to say more on the subject, but I feel that this should be enough to give you an idea of how to discuss morality with atheists. I hope that this helps!