Wow that is a tough question and has been debated much on these blogs. My personal opinion is that YES you can indeed improve your score on the GMAT with practice and guidance, especially if you were severely underprepared or are in general not a good "test taker." I would highly recommend a prep course in your case since you are trying to improve so dramatically. Having said all that, it's not true that everyone is capable of a high GMAT score with enough practice, so if you were to take a prep course and adequately prepare and then still not see much improvement, you might be hitting some resistance at the top of your range. It's way too early to make a judgement on how well you could potentially do, however until you adequately prepare and take it again. I will say that in general, properly administered practice tests are usually good indicators of how well you will do on the real thing (within reason), so don't go sit for the real thing again until after you prepare and are getting at least close to your target range on full practice tests. Good luck.