You can normalize you MP3's as you burn them with "MP3 CD Maker" if you turn the normalizing switch on.

I've also used it to adjust the volume on a single MP3 by telling it I wanted to do a test burn, then aborting the process after the normalizing phase.

You can download it at: www.zy2000.com

If I also want to edit the MP3, I use Wave Studio to edit it and adjust the volume, then save it back to MP3 format (or just burn it from the wav file).