I am not sure what is the maximum that your laptop can take... but if you have 533 right now, it is best to buy the same speed RAM if you are going to use the current RAM and wanted to add some more...
However, if you are not going to use the current one and wanted to upgrade to the max speed RAM your laptop can handle, you would then have to remove the current 533 RAM and replace it with whatever max speed RAM your laptop can handle...
Hope I am not confusing you...