This is a tricky question. Scores on practice tests are just an indication of your performance and are in no way an assurance of your actual Test Day results. There are so many factors you need to count in to simulate a Test Day like situation and more often than not, we fail to do so. In general, Test Day experience tends to be more stressful and difficult than the practice test sessions. Also, the score from one test should in no way make you confident about the final score you are going to get in the real Test. You need to take at least a couple of practice tests to roughly assess your level of performance.

For a straight suggestion, I will suggest that you take one of the GMATPrep official practice tests and see how you score. Thereafter, you should analyze your test performance, check the answers and the explanations, mostly for the ones you got wrong or right for the wrong reason. This will help you identify the areas you need to focus, both in the Quant and the Verbal sections.

People tend to argue that, if you score at or above your target score for thee-four tests, then you can go ahead and take it. A 500 is not that hard to get, especially if you've had some decent contact with math and if you're not usually nervous on test days.

I think becnil's advice is pretty good: GMATprep is very accurate and will most likely be a true indicator of your current ability. For instance, I took GMATprep a few days before my test and scored a 760. On D day, I managed to get a 770! That's pretty much spot on in terms of accuracy!