Programs like Audio Grabber and Audio Catalyst are both excellent. You can use their default codec or import your own into the program. They also allow you to encode your mp3's anywhere from 16 bit up to 320 bit for superior quality. Hope this helps
By the way cakewalk 8.0 and 9.0 have the ability to encode files into realmedia files for streaming as well as MP3. Other software that comes with soundblaster sound cards give you the ability to record directly from CD or Analog input and convert it into MP3 format. Please check their website for details.