AAC is a general codec like MP3. It can compress the file size as much or as little as you like. At 320kbps, you are compressing at 4.4:1. At 256kbps, it's 5.5:1. At 128kbps, it's 11:1. It's your choice, if you're the one doing the encoding (e.g., if you're ripping from CDs with iTunes). If you download from the iTunes Music Store, you're getting 128kbps.
There will be a clear difference between CD and 128kbps AAC. You'd be hard-pressed to tell CD and 256kbps apart without knowing which was which unless you know exactly what to listen for.
As for Apple Lossless, Apple claims 2:1 compression, but the tests I've seen suggest that, on average, you don't get quite that much. A typical Apple Lossless file will be somewhat more than half the size of the original WAV or AIFF file.
