The quality depends on the source mp3 files.
Most of mine are 320kbps, but I've had good results with 160kbps too.
If you're going to be listenig with headphones, go with a higher bitrate mp3 as your source.
If you're making a disc to listen to in the car, 128kbps should be fine too. You'll probably never hear the difference over the road/car noise anyway.