Jared is spot on with his advice. Follow what he said, but I do recommend reading the entire book, so you can see some of the other craziness in there. The book of Song of Songs gets really intimate and romantic, it's more of a collection of poems, if you ask me. Proverbs will almost inevitably be used at some point in an argument. Leviticus is the basis for every homophobe, 18:22 says, "Do not lie with a man as one lies with a woman; that is detestable." (NIV) It also talks about beastiality and other sexual no-no's. Well, haha, I hope you come to a decision and get to reading. It will certainly affirm your atheism.
I'd also if in English would read the King James Version. Not because it is more intelligible or for it's literary value, because it possesses neither in discernible quantities (or I consistently accidentally picked out the wrong paragraphs.) But because the effort to interpret old bullshit in newer terms just adds fresh bullshit on top of it, usually unsupported by anything but current fashion in theology land.
Currently I'm letting it read to me on my ipod (while at work usually) with enlightening commentary by http://www.thomasandthebible.com/
I have the English Standard Version on my Kindle as a quick reference, the translation is OK (though it is arguably evangelically leaning and fails the Isaiah 7:14 test) and it is easy to read (also it was a free Kindle download). In general though, I don't encourage anyone who wants to actually study the bible to go solely with a digital e-book version as you lose a lot that way in the way of annotations and ease of reading when it comes to doing parallel readings.
For the Bible in general I suggest the Oxford Annotated Revised Standard Version, College Edition. Get a print version not a digital one. The print version has annotations that deal with translation issues, omissions, commentary, etc. For the Hebrew Bible/OT I suggest the Jewish Publication Society translation.
If you want a good digital version and are serious, shell out the money and buy an actual bible software. I use Bibleworks 8, Logos is arguably better (albeit it only works on Macs and is much more expensive). It will allow you to search through multiple translations, read commentaries, search through a thorough critical apparatus, and check translations against grammars and lexicons. If you are not willing to shell out the money, E-Sword is free but is nowhere near as good and some of the add-on downloads cost a few bucks.
NSRV - this is the most meticulous translation to modern English. The maniacal academic focus on preserving the words 'in context" while still retaining the poetry. I can almost always talk rationally from the text and about the text. NSRV concordance version would be a win. Most foolish mistakes 'Jesus was born of a virgin' v 'Jesus was born of a young woman' are fixed. [citation needed, this is a type of category error addressed even if this one example is wrong].
This is probably the least popular version because it nullifies much fundamentalist dogma. If your goal is holding your own against a chrisianist - forget it
1. They won't listen anyway
2. you are not using the 'real' KJV or NIV which is what evangelicals/fundies acknowledge as the version blessed by Jehovah. Yahweh is a white, 16th century European deity after all.
And when you have finished with the KJV or NIV or whichever you chose, head on over to the Lolcat Bible Translation Project at
Same crap, but reads much better.
My first recommendation would be the New Revised Standard with a decent commentary in hand. My second recommendation would be a copy of the New American Standard with a decent commentary. Both cling relatively close to a literal translation, though the NRSV in my experience tends to be more readable and less likely to cause the reader to throw it against the wall and thus more likely to see you through.
HarperCollins' study bible with Apocrypha or the New Oxford Annotated would be ideal, and can both be gotten CHEAP (like $2.00) used. Both the HC and Oxford Study Bible provided a lot more context for the individual books, including timelines, maps, and historical-critical analyses that are necessary in understanding what the authors might have intended, while decidedly staying away from an evangelistic focus. I can honestly say you'll understand the Bible and its background better than most Christians with one of those two texts.
KJV lies third, though as many people have pointed out it still has cultural value and a very strong following within the church.
No matter what, I'd suggest a decent commentary or dictionary - don't buy a concordance unless you actually plan to learn Greek and Hebrew. If you live in a town with a decent (or Bible) college, the Anchor Bible commentary and reference series show a compendium of the best scholarship available. If not, the Anchor Bible Dictionary, which "includes articles on the Dead Sea Scrolls, early Jewish-Christian relations, the historical Jesus, sociological and literary methods of biblical criticism, feminist hermeneutics, and numerous entries on archaeological sites, as well as bibliographies with citations listed individually at the end of each article" is available on CD.
A lot of good advice here. It really depends on who your 'target' is, who you have in mind when you're arguing.
If you have a particular denomination or related set of denominations then you're best served getting one of the translations that they use. So if you're trying to target evangelical christians, you should get the ESV or NIV. The NIV is also a good choice generally in terms of being familiar to North American christians. In any event, it's easy to call or visit a church and find out what the pew bible is for them.
If you really want to play it safe however, the KJV is the best option. While all translations have their detractors, in terms of avoiding debates over the translation you chose...KJV will take you the further with more denominations than any other.
Lastly if you can get a study bible or annotated bible - do. Having more information about the bible and the beliefs of christians can only help your position. Having such a resource is invaluable because it means you can get the sophisticated interpretations up-front and thus be well prepared. I don't know if your argumentation style involves being able to beat people on their own turf...but if it is, study bibles will help you greatly in that task. If you'll be engaging with serious catholics it would be prudent to get a version that includes the apocrypha.
Whatever version you decide to get, you might want to consider also taking a look at the NET [new english translation] bible online. It's freely available and they do a good job of explaining their translation decisions. That resource will provide you with a good insight into the translations choices that have to be made...and it won't cost you any money.