There is some information missing in your scenario -- specifically the amount of rent you were to pay each month, and the move in date on your contract. That aside, here is one attorney's view:
If the amount you would owe in rent from the day your contract said you would take possession until the new tenants rented the apartment sometime in September is greater than your deposit, you are not due any of your deposit (monies owed for rent can be taken from the deposit). If that amount plus the cost of advertising the availability is more than $1600, then she can take the money.
HOWEVER, within 21 days after move out, the landlord is REQUIRED BY LAW to give you an itemized statement of what the deposit was used for (along with receipts where applicable). Your move out date is the date the other tenants took possession in September 2008. If you did not receive at least a statement (or letter or some other written communication) explaining what amounts were deducted from the deposit, then you would be correct with your numbers of $4,800 (treble damages) as the amount of your suit.
So in short, it depends on some additional facts that are not provided.
The above is not legal advice nor does it establish an attorney-client relationship with the person posting the question or any person reading the above. There is no attorney-client relationship between any reader and this writer or her firm. These comments are not subject to any privilege protections. They are neither privileged nor confidential. The information is general only and should not be relied upon in any specific case. Accordingly these comments cannot be relied upon as the law and the facts may differ from those with which the reader of this communication may be dealing.