I have a solution for those unsure on what bit rate to encode your AACs at -- always import at a relatively high bit rate; I use 192 kbps.
If you later run out of space and were wishing that you'd imported at a lower bit rate, you can simply select a lower bit rate in iTunes preferences and select "Convert Selection to AAC" in the Advanced menu. iTunes then downsamples your music to the lower bit rate and you can fit more into the same space.
[robg adds: The only downside to encoding at a higher rate, whether it be MP3 or AAC, is that you'll (obviously!) be able to fit fewer songs on your portable music device. That, and it will be a bit of a pain to re-sample 2,500 songs when you run out of space! Personally, I've chosen to encode at 192kbps, but I've also stored my library on a separate hard drive inside the machine ... if it ever fills up, I'll just buy a new, larger drive and move everything across.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030610031138827