With the amaroK ABX-Comparator you can determine the perfect audio codec settings for your personal equipment (including your ears...).
First choose or create two audio files. One file should be the reference and the other the challenger ( the "A"-file and the "B"-file),
for example FLAC against vorbis q6 or mp3 at 160 kbs versus mp3 at 128 kbs and so on.
A good source for listining examples is www.ff123.net .
Then one of the two files is randomly chosen (the "X"-file). You can now listen to the 'A'-file and the 'B'-file and try to determine the 'X'-file
(That's why it's called an ABX-test.). To avoid sheer guessing you have to do this several times. After your "probability that you were guessing" is sunken under 1%, you could be fairly sure, that there is no difference for you between the two files.
So, if you are still encoding your mp3s at 1000 kbs, ABX it out if you hear a difference to ogg vorbis at q6!