I would suggest you take GMATprep 2 and stay with your goal to take the test on the 14th. But it is your moolah! The GMATprep's are the best gauge of your actual performance! If you feel you need more time, cancel. But read over the topics for study strategy on this forum. I'm relatively new to this site, and I've already found its content priceless!
In the case that you decide to take it as planned, remember: A lot of your prep is mental! Don't psyche yourself out! Work on your pacing so you make sure you finish each section. Look for explanations on the areas that are giving you problems or just taking you longer to solve.
Then get a good night's rest (good luck with that--but remember, it's all mental), eat a good breakfast, put on something comfy, and relax while you're in the test room.
You'll do great! Just tell yourself "I WILL
score 650! I know this stuff!"
It sounds cheesy, but so does letting the GMAT intimidate you!
Keep the winning attitude and let me know how you do!
Best of luck!