Remember you will only be able to change the Region Code in your computer 5 times. After that it will be locked to the last Region Code you used.
So if you have DVD's from say Japan and the USA, then each has a different Region Code. If you play a USA, then a Japan DVD, that will use up one of your changes, then if you go back to USA then another will be used. Once you use the 5th change your machine will only play discs from the Region Code you last set it to. SO you could only play USA or Japan NOT both.
This has nothing to do with NTSC or PAL as they are not Region Code based.
They are simple the format of the TV signal being output.
If for instance you only ever use Japan DVD's all with the same Region Code then it does not matter if they are PAL or NTSC you can play them as many times as you like, no issue as you swap from one to the other.
Region Codes are there to prevent content from being bought in one region of the world and played in another.
You can read more about Region Codes here :-
http://en.wikipedia.org/wiki/DVDregioncode
Ian
