From the schools that I looked up (Tufts, UPenn, VA-MD, Illinois) it seemed that the average GRE scores for attending students were Verbal- ~630-650, Quantitative- ~680-700, Analytical Writing- ~5. I would agree with youthman in that the average scores (or a bit above) would be a good measure of "strong" GRE scores for your application. You may want to look up the averages for schools that you are applying to also. Hope this helps!
Lisa