I recently did some listening tests concerning MP3 bitrates and encoders. LAME at the highest VBR setting is the best that I'm aware of. It eliminates 99% of audible artifacts, even on exceptionally difficult tracks. Basically, with that setting, I typically get files ranging anywhere from 170kbps to 300kbps with an average of around 240kbps. The encoder determines the difficulty of each track and encodes at the bitrate it determines is necessary to make the MP3 transparent. The only setting higher than that would be a flat CBR 320kbps, but the improvements in sound quality are arguable and you're essentially just wasting space at that point. You can get good LAME encoders on PC, Mac, and Linux.