680 GMAT, 3.4 GPA. Retake GMAT

In a word: yes. For applications to top business schools, it's always advisable to be the strongest applicant you can be in every area. If your GMAT is below the school's median, that places you at a competitive disadvantage relative to other applicants.
I sense from your email, however, that you don't feel you can necessarily get a higher score. There are many other factors to consider: age, geography, industry, undergraduate institution, professional history, not to mention the Q/V breakdown... you may have other strengths that will help bolster your competitiveness. An effective MBA application consultant can help you develop a big-picture strategy that makes the most of these strengths while neutralizing any weak areas as much as possible.

That said, if you can retake the GMAT, do so -- even if you don't get an improved score, the schools will see your commitment, and that counts too. Good luck!