i believe (but am not sure) that if you've already ripped the tracks, then it's too late to normalize them.
i do recommend if you decide to normalize to do a few test tracks first and see if you hear a difference.
i didn't like normalizing the mp3 files because it tended to compress the dynamic range too much for me. the songs sounded kinda muffly. i figured since i was already compressing, i didn't want to "make it worse" by normalizing.
Ted - Good point about the nmormalizing. I think I was kind of lucky on my cd where the majority of the songs are pretty much on the same level. A few seemed pretty low so I think I would rather have a couple of songs lower than the other than the other way around. It would blow the speakers!
It depends on what method of normalization we're talking about. Normalizing by "peak" level doesn't alter the dynamic range. It doesn't give as consistant a volume as "average" level, but it can make the differences less extreme without damaging the recording.