I'm not quite sure if you want general arguments against christianity or a defence of agnosticism!?
I would say there are at least three kinds of agnosticism. 1. The ‘I just don't know’ kind. 2. The more aggressive position that there is not enough evidence so the right position to take is agnosticism. (this seems to be CE’s position) 3. The rare and extreme position that it is in principle impossible to know if god exists.
I personally think all three positions are wrong. at least when it comes to a christian, muslim etc. god because there are enough reasons to believe these kinds of gods don't exist.
But I am an agnostic when it comes to the notion of a creating god. So if the question only concerns a completely featureless god who created the beginning then I have seen nothing convincing for or against. But this is not what your missionary friends are talking about - they are arguing for a god with a whole lot of features.
I think that the best arguments against a christian god relies on the fact that he is all good, all knowing and all powerful. The problem is that there is a lot of suffering and evil in this world. But if the christian god really exist and have those features then it becomes a mystery why there is all that suffering and evil. You can have a look here for more.
http://gamebanshee.com/forums/showthrea ... adid=13737
There is quite a bit in this thread and Eminem and I discuss this and other problems.
If you want more than this then try the ‘Blackwell Companion to Philosophy of Religion’ I have never seen it but I have other Blackwell companions and I think they are excellent so you should be ok. If you feel like you don't want to spent that kind of money a good trick is too find a good academic book shop read the passage and go - they wont mind.