You can only upgrade to the current version which is CS6.
Unfortunately the upgrade window for CS3 and CS4 users closed on 31 Dec 2012 after being promoted by Adobe for 12 months.
Only CS5 and CS5.5 users are now eligible to upgrade to CS6.
Your options now:
1) Pay full price for CS6
2) Join the Creative Cloud and pay the monthly fee (monthly or 12-monthly contract)