I don't think it's worth retaking the GMAT. As long as it's still valid, you should be ok (I remember GMAT scores have a validity of 5 years, I don't remember if they should still be valid by the time of application, application deadline, enrollment or something else).
730 is a great score, and improving from 50Q to 51Q shouldn't make much of a difference. With a score like that, I think schools will not be concerned with the GMAT anymore, and they will focus on the other parts of your application.
Please remember that top 20-30 is typically very competitive, and you said your profile isn't too good. Applying widely is usually a good strategy.