You may deduct only the student loan interest that you actually paid. The actual amount of student loan interest should be reported to you on the form 1098E - http://www.irs.gov/pub/irs-pdf/f1098e.pdf -you may not deduct mor e interest that you actually paid - if you do - the IRS will likely notice discrepancy and would ask you for explanation.
for details on student loan interest deduction - please refer to the IRS publication 970 - http://www.irs.gov/pub/irs-pdf/p970.pdf - page 25
If your student loan interest amount is more than $2500 - you may not carry over the difference to the next year - that deduction would be lost.
In additional, the amount you may deduct may be phased out or eliminated based on your filing MAGI. if your modified adjusted gross income (MAGI) is between $55,000 and $70,000 ($110,000 and $140,000 if you file a joint return). If your MAGI is above the range of incomes - you cannot take a student loan interest deduction.
You will deduct $2500 - student loan interest above that amount is not deductible.
That limit stays for long time - it is possible that it will be increased.