I would definitely retake the GRE until you break 700 for both the Quantitative and Verbal. The GRE won't get you in, but it can definitely keep you out. PLUS, it can add upwards of $6,000 per year to your stipend depending on the program. I GRE is a bear. I took it three grueling times. But, on the other side of the application process (well, almost on the other side), I can say that taking the test multiple times was the best decision I could have made. Without doing so, I would not be in the position that I am currently in. I am certain of that.
Hope this helps...
Mike