Sounds like you are just starting on the GMAT journey. I would recommend visiting this post:
gmat-study-plan-start-your-gmat-journey-80727.html to help you with your study plan.
Also, the more you take the GMATprep tests from mba.com, the more repeat questions you will get. So eventually your scores will not mean anything b/c just a few repeat questions can boost your score SIGNIFICANTLY. I would recommend that you purchase a
Manhattan GMAT book to get there free tests, or sign up for a course such as Kaplan.
Also, there is a huge pentalty for not even attempting questions. It is not advisable that you ever leave a questions blank. It would be better to guess on all the last questions than to leave them blank. Timing is a huge part of the GMAT, and you should be able to get to every problem. A general rule of thumb is 2 minutes per quant questions. Obviously it is great to string correct answers together, but if it costs you a long string at the end of a section, your score will suffer badly.
A 680 is no easy score as it is in the 86th percentile. It is hard to say what score combination you need b/c you can do very poorly in one section and great in the other and still score a 680. So there is a large range of scores in both sections you could get to score that 680 you need. See the scoring matrix here:
gmat-scores-83890.html and you can see what kind of score ranges will get you what you need.
As far as the essays are concenered (They're called AWA), they are a totally separate score. They are scored out of 6, and are not really that important. You just HAVE to score a 4 or higher otherwise it could look very negative on your application. I would suggest you don't spend much time on these, but DEFINITELY practice them at least a few times. You can find great templates for the AWA questions here:
how-to-get-6-0-awa-my-guide-64327.html. GMAC (the company that created the GMAT) is chaging the format of the test come June 2012. They are getting rid of one of the AWA essays, and adding in a new section called Integrated reasoning. It is suggested that you take the test before then, or you will need to study for this new section. The scoring for this new section comes out in April. If you have any questions about this new section see the GMAC website here:
https://www.mba.com/the-gmat/nex-gen.aspx.
If you have any further questions, come back, look around, ask questions, and enjoy your journey.