I'll have to disagree with mohater. I think you should delay the exam if you feel like you haven't adequately prepared for it. Don't depend on an adrenaline rush to get you an extra 50-100 points. Agreed that some people do better under pressure and some people don't. With that said, even if you are someone who performs well under pressure and get a lot of studying done, I don't think a 600 is going to be a 700.
Yes, taking the GMAT will give you an concrete idea of where you're at, as test prep materials are often more difficult than the real thing, but you can get that by taking the exams on the official GMATprep. Taking the GMAT when you're not ready for it will likely result in a low score and loss of $250. Plus, schools, even though they say they will only take the highest score, will see all of your scores. My advice is to spend some more time preparing, and when you feel like you're ready, take it.