You don't need the burning crusade or lich king expansions yet, all that will be missing is being able to play as a Blood Elf or Draenai (spelling)
At level 58 you will be able to enter Outlands, which is the Burning Crusade expansion, then at 68 you can go to Northrend (lich king expansion)
there maybe a few more small details i'm missing but thats about it in a nutshell.
Not sure how the upgrade works ( i have all the disks), but i pay £8.99 a month, so i'm guessing that the upgrade you did was for a months play time.
*edit*
take a look at this thread, OcUK alliance guild just starting out, might be worth a look as most are re-rolling experienced players.
http://forums.overclockers.co.uk/showthread.php?t=17950643