It's actually very easy to move your iTunes library to an external hard drive.
http://support.apple.com/kb/HT1449?viewlocale=en_US&locale=en_US
Best thing about it is the fact that you don't have to move anything when you change computers in future.
I recommend making a backups of the external drive though onto two extra external drives. I keep one backup in the garage and another in a relative's garage. The last thing you want to is ripping your CD collection again. (One backup is no backup).
A couple of years ago I had a hard drive failure with my MacBook and I was very happy that I had an almost complete back up at home when I came back from holiday.
Another very handy feature is the fact that I can take my iTunes library on holiday and use it with my old MacBook, rather than the MacMini at home.
Keeping the ripped CDs as AIFF is a complete waste of space, IMHO. I would convert everything to Apple Lossless (ALAC) and rip everything you haven't ripped as ALAC. It will save you halve the space on your hard drive.
I just looked and Tunespan is available in our App Store for €13.99. I would recommend though to keep your whole library on one disk if it's under 1TB.