I noticed an mp3's quality depends largely on it's bitrate, the codec used to mp3 it, and the quality of the machine it was mp3'd on.
Oggs tend to lose quality in it's compression but tend to keep goo high and low end tones, it's the midrange on them that suffers.
I have a few mp3's that have a bitrate of 320 that I 'ripped' myself using Winamp 2.91 and the Lame encoder, and you can't tell the difference between them and the original CD.
At around 196 bitrate you will notice differences..
People with better ears than mine can notice a difference at 225 bitrate.
*sigh*
But I can hear dog whistles.
I bet, if you haven't already seen it, that you'd like CDex.
http://www.cdex.n3.net/
It's a heckuva good ripper, and handles all sorts of mp3 variations, and it's a freebie, (though as always he appreciates donations).
Beware of NeoAudio, a blatant ripoff with spyware added.
"Great, free software!"
I wish everyone would stop complaining about spyware! Is it really that much of a problem?....