Probably worth pointing out that, once you've changed your settings as Dave says to Apple Lossless, if you right click on an existing 320 Kbps file, it will give you the option "Create Apple Lossless Version" - don't be fooled! All this will do is create a larger Apple Lossless version of the 320 Kbps file you have - it won't be a lossless version of the original recording as you can't put back the detail which has been lost.
So, as pointed out, you will have to then re-rip all your music once this setting has been changed from the original CD. Depending on the size of your music collection, this can be an arduous task (took me a week on and off of adding and removing CDs in and of the tray!). However, if that seems like too much, it might be easier just to do your favourite CDs first and then, as you generally want to listen to a CD, quickly rip it first (generally only takes a few minutes) and then slowly build up the lossless collection that way.