(1) The ARCO is a good book, but very, very basic. The math on the GMAT is significantly harder. Skip it if you are already familiar with the concepts.
(2) The Kaplan GRE/GMAT workbook provides a very solid foundation for most problems, although it does not sufficiently cover probability or permutations. Many people seem to encounter anywhere from one to three probability/permutation problems. Make sure you pick up a high school textbook, or some other source, to learn these skills.
(3) Regarding both the OG and Kaplan, there is an excellent piece of advice that I actually read in someone elseтАЩs posting on this Forum. When you get a problem wrong, return to it. Do it slowly. Change the numbers or parameters to make a similar but new problem, and try solving it. Also, keep a record of any problems you get wrong, and return to them after a week or two. In other words, when you get any single problem wrong, review it and repeat it until you understand it. Basically, if you do the problems in the OG and Kaplan, and you тАЬunderstandтАЭ at least 95-98% of the problems, you will probably do quite well on the GMAT quantitative.
(4) Also, when you are trying to solve a problem that you donтАЩt understand, take the time to try to figure it out yourself before just looking up the answer. There were five or six problems that I probably spent about twenty minutes on each, and finally I was able to answer (almost) all of them. By slowly coming to the conclusion myself, it really helped me master the process/concept. It forces you to use your own brain. Of course, this type of study depends on how much time you have to study.
(5) If you find yourself pressed for time, work backwards in the OG. The harder problems are towards the end. Remember, keep track of any problems you get wrong, and return to them. Keep studying these problems until you understand the underlying concept.
(6) Finally, it is important to consider timing in your practice and study. This is why I recommend taking as many practice quantitative exams as possible. Alternatively, when you study, you can time yourself. Trying doing blocks of twenty questions in 35-40 minutes, or less than two minutes per question. The first time I studied for the GMAT, I did not practice this type of discipline in doing the questions. Then, on my first GMAT, I panicked and ran out of time. The GMAT is not only about knowing the concepts and doing the problems, but doing them in less than two minutes each, on average.
(7) One final note: Kaplan teaches a few тАЬtricksтАЭ to solve problems, like substitution or back-solving. This can actually be helpful. There were probably one or two questions on the GMAT that I could not do the math, but using these tricks, I was able to arrive at the answer. I prefer tricks as a last resort; ideally, you should know the math and underlying concept. However, you should also be familiar with the tricks. As you do the practice problems, try using formal math first. Then, see if you could have applied one of the tricks also to solve the problem.
I hope these miscellaneous bits of information help. The GMAT is not that hard to beat. I was not able to тАЬaceтАЭ it, but through lots of study, my math score is places me in the top fifth, and my overall is the top 1%. It took a great deal of effort to learn the language of math, and to develop the confidence to tackle the math problems. But itтАЩs really just a matter of resolve and practice.
Good luck!