Hi hartattack, don't worry about your test score and just focus on how to improve. To score a 550 or above, all you really need is an OG edition. The OG questions are real questions from past GMAT tests and will simulate up to a score 650 with the hardest questions. Since you're aiming for a 550 (or above), OG itself should be sufficient enough provided that you understand exactly why you miss a question. Take a few practice test from GMATPrep. MGMAT books
are very helpful; however, at this point and the score you're reaching for, I can't be too sure that you'll need those extra books. Having said that, if you do decide to get the MGMAT books
, use them to your advantage. These books are very detail-oriented and should carry you up to at least 600 if you're being true to yourself.
Good luck! and visit this forum for any questions you may have or when you have extra time, just stop by and do a couple sentence corrections