Whatever encoder you're using should have a variable bit rate (VBR) option, which you might want to try. VBR encodes those parts of the music requiring a higher level of recorded detail or bandwidth (think high notes, cymbals) at 256 or 320, and those parts requiring less bandwidth (low, bass tones) at 128/160. The bitrate within the resulting mp3 usually switches several times a second, giving you better sound when you need it, without increasing the overall file size needlessly.