How can I find out how much memory is left on my ipod?

I put lots of music on itunes, then put it on the ipod, but ran out of room on the computer memory. I dumped whatever I could and then imported more music, but then ran out of room again. Finally I dumped my music library and itunes itself and started over. When all the music was on itunes the first time the display at the bottom of the screen told me how many mb's of music I had, plus how many songs (and "days", whatever that is). I have a 4GB ipod and was getting close to 3MB. But now that itunes has only a few CD's on it I need to know how many more songs my ipod will hold, or in other words how many MB's does the ipod have left? And- what happens when it gets full? Does it automatically start dumping songs in order to make room or will it just tell me it is full?