So your iBook's hard drive is full, but you have an external 350 GB drive that has free space. The previous poster's suggestion is a good one, and here is another idea. Move a bunch of your music out of your iTunes music library (the folder on your drive, not the line items in the iTunes window) and onto the external drive. This will free up space on your iBook's drive and let you keep as many songs as you want on the external drive. iTunes will still manage them so you'll be able to put them onto your iPod no problem.
Here's how it works. You define to iTunes a folder that it calls your "iTunes library". When you use iTunes to rip music from an audio CD, the resulting MP3 or AAC files will get stored inside of this folder. The folder is usually located in /Users/your_user_name/Music/iTunes. However, iTunes can track and manage music files located anywhere on your computer; they don't have to be inside of the music library. So you could store 5,000 songs in a folder on your desktop, drag the folder into the iTunes window, and it will define those 5,000 songs to iTunes. Just make sure that in the Advanced section of iTunes Preferences you uncheck the option to make copies of all songs added to iTunes.
So if you relocate your music files from inside the iTunes music library folder on your iBook to a different folder on your external drive, that will free up disk space on the iBook's drive. Then if you import a bunch of music from CDs there will be room for them. If you are adding a bunch of songs that are already in MP3 or AAC format then just place them on the external drive and then drag them into the iTunes window and it will add them to iTunes. Then you can sync as usual.
Does that make any sense? The difference between this idea and the previous one is that the first suggestion moves your entire music library to the external drive, which means that iTunes could get into trouble should you launch it without the external drive connected. The benefit of this method is that all your music is physically located inside of the iTunes library folder. The method I suggested keeps your music library where it is, but relocates some of the music files to the external drive. This means that your music will be stored in two different places, but if you launch iTunes without the external drive connected then the only downside is you won't be able to listen to those unavailable songs.