Kaplan is good practice but not enough by itself...also get
the Official Guide (from Amazon.com or mba.com) and Cracking the GMAt (the one with the CD), which is from Princeton Review. I prefer PR's strategies to Kaplan, but see how Kaplan's work for you since you have the book and then try PR if you need more help. PR is very good on verbal.
Also be sure to download the free Powerprep tests on mba.com.
btw, don't worry about the practice test scores you get on Kaplan tests. They're well known for being a lot harder than the GMAT and will underestimate your GMAT score by 50-100 points. PR's and the Powerprep tests will give you a better indicator of what your score might be on the real thing.
As far as whether you need a 700+ to compensate for a low GPA, hard to say. I would imagine that b-schools might give more weight to your undergrad grades than they would to med school grades (unless perhaps you are going for an MBA concentration that's related to health care, maybe). If you didn't take coursework in business topics or didn't get As or Bs in math classes in undergrad, then it might help your app if you take some business-related classes now - finance, accounting, econ, business management. Get As or Bs in those classes, and it will help your app (if you're applying for Fall 2004 you won't be able to do that beforehand, but if you register for a few classes in a local college for this spring semester you can refer to that in your app so the schools know you're getting yourself prepared for b-school).
Hope that helps!