The amount you will need to live on really depends on many variables. For instance, what part of the country will you be in? NYC is far more expensive than say Phoenix. Will you be on a meal plan? If not, then you will need to calculate a good amount of money in for groceries and/or eating out. Also, do you go out often? How often do you like to shop for clothes? Are you living in a dorm or in an apt? Do you plan on having broadband internet access? What about cable tv? Do you have credit card bills you are paying down? Are your utilities included in your rent?
For one person in a large city, you should budget $50/week at the supermarket (this is an average... you can spend a lot less of a lot more depending on your eating habits).
A good way to figure out how much you budget would be to go back and track for the past 3-4 months. Use your old bank statemend, credit card statements, receipts, etc and see where your money has gone. Get a program like Quicken or Microsoft Money to help you out.