Yeah, many gods are logically impossible (mostly stemming from being simultaneously omnipotent, omniscient, and all-loving). The concept itself isn't (see: Deism). Neither are all specific gods (pantheons can often be rationalized by the fact that no member is omnipotent and there's a general tendency toward selfishness, so that failures to act on behalf of or against humans are easily explained by their complete lack of concern).
Overall, though, it seems to me that you have to take different stances with what you believe, and what you think other people need to believe. It's one thing to determine that you believe there is no God, but it's another to insist upon that to somebody who does believe it. I think a complete lack of evidence is sufficient for my disbelief, but is insufficient for undermining somebody else's beliefs. Especially because, as far as they're concerned, their life experiences have provided evidence, and no amount of debate can convince them that a lack of evidence that God does not exist will trump their own experience that suggests he does. The best you are likely to do is get somebody to admit that God may not exist, but probably does. That's my experience, at least.
Also, I apologize for my laxness in paying attention to this thread. Arguments always seem to happen 2 hours before I get on the forums and then wrap up before I have a chance to say or do anything. It's good that they end quickly, but I hope they occur less frequently in the future.