If you have an external drive , use it to backup your MBP using Time machine. then when you install your new 500gb drive , boot up from a leopard install cd or the install cd's you got with the MBP.
Go into Disk utility , partition your new 500gb drive how you want it.
Then choose recover using time machine from the drop down menu ( where disk utility was ) and restore your time machine backup to the 500gb drive , job done.