Duplicating Part of Itunes Library on Powerbook from External Hard Drive.

Hi.
My 40gb Powerbook is filling up with my itunes library so I'm about to buy an external hard drive. When I do this I'll put my itunes library (approx 20gb) on it to free up the Powerbooks Hard Drive. Although i've not done this before I'm led to believe it's pretty straight forward to redirect the library to pick up the music from the ext. HD (but please tell me if I'm mistaken!).
Here's my problem though:
Whilst I'll happily download and rip any future music onto my main library on the Ext. HD I'd like to keep a core selection of about 3gb of songs on my powerbook(but also have them on the Ext HD) so that I always have some music wherever I go. Is this possible?! Maybe there are scripts I could run (but i don't know too much about scripts either) so that I can have 2 separate librarys or rather a complete library on the Ext HD and some selections from it on my laptop.
Many apologies for the bracket overload!
Any help would be greatly appreciated.
Thank you in advance.
Ronan

There is an easy way to do this.

When you move your iTunes library to the external drive, create a second folder called "Core". Put the Core songs in that folder, and the rest into your iTunes Library folder. Now, copy the Core folder back to your main drive.

Delete all of the songs from iTunes (Highlight all songs, Delete, Yes for delete the songs, no for delete the files). Now go back into iTunes and select "Add to Library" and choose your iTunes Library off the external drive. When that is done, select "Add to Library" again and choose the "Core" folder from your Main drive.

All of your songs will be in iTunes now, but you will have to keep track of which are core and which are not. I would suggest creating a playlist in iTunes called "Core", that way you will always know and adding all of the core songs to it. This would be easier if you added the core songs to your Library first, built the playlist, and then added the songs on the external drive to the Library.