Each CD ripped, depending on which format you use; if you rip Lossless or to a .wav or .aiff encoding, is going to only occupy at most 700MB of space (Lossless, .wav, .aiff) and usually something way smaller, such as 70MB (ACC/mp3.) You should be able to move the folder right away after it's ripped to the external drive. In iTunes right(control)-click the song and select 'show in finder' from the context menu to find where the folder is.
Keep in mind that if you are dragging a folder from the internal drive to an external, Finder won't 'move' the files, but copy them so you have to trash the folder on the internal drive.
You might be best served if you move your iTunes library to the external drive. I used to have a Mac Mini and thus had an external drive always connected so I kept my music on that drive. Now that I have a Macbook which is mobile, the external drive isn't always connected so I don't keep my music there, but it's entirely possible to have iTunes keep your music library (and thus everything you rip from your CDs) on the external drive. If you're mobile though, you won't have your music with you everywhere you go.