Good questions! Luckily I just wrote an article about this:
http://www.manhattangmat.com/blog/index ... -the-gmat/Biggest takeaways: don't set a timeframe for the re-take right away. Dig into your studies and see what this is going to take.
After you've read that article, come back here to ask additional questions. Re: your question about materials, in addition to OG, you need whatever materials will cover your weaknesses. Not sure what those are? Read that article above to learn how to figure them out. :)