In my opinion, I think its worth to put some extra effort and try to take GMAT once more. Since, you already have a very good English background, you can easily improve your score. You just need to sit down for few weeks and practice the kind of questions GMAT asks.
This should boost your score beyond 600 and then you can get into top-20.As for the importance of the ranking of the university -- I think coming from a very good school gives you the initial boost to get placed in a good firm. And I think school does matter when getting placed right out of college. Thast what my experience says -- though I am from engineering background. Even then, when I interview ppl, I do check the college name.
Good luck!