andres: First of all, don't be discouraged! Reading through your post it seems like English is not your first language. If that's the case, you need to put a great deal more time into preparation than the people who "got 690 after two weeks." That being said, plenty of ESL students have scored very highly on the GMAT. It all comes down to how dedicated you are to following a study plan and putting in the necessary effort.
You mentioned that you were disappointed with a 530 after a 520 and a 540 on GMATPrep. That should not be the case. You should be happy to score around your practice test scores on game day, that's all you should really hope for. Therefore, you should not take the test until your practice scores are right around where you would like to be on the real thing.
If timing issues after running into hard questions is your main pitfall, I would read this thread:http://www.manhattangmat.com/blog/index.php/2011/06/21/everything-you-need-to-know-about-time-management/
Basically, if you're running behind, SKIP DIFFICULT QUESTIONS. I would also search the forums for people's "debriefs" to get a better idea of how to prepare.
Best of luck and I hope that helps!
My GMAT Experience: http://gmatclub.com/forum/gmat-debrief-770-q50-v47-115912.html