Poster: tairip
The quality that yout have to use as a pro DJ is: 320 or highger! Or WAV!
The rest is onacceptable, That's my opinion
It really depends on the complexity of the songs. 128 kbps can become a problem for complex sounds and than a user can notice the audio quality. However in a club there is to much noise most of the time to notice this. But you can test it yourself. Playback the original and the Mp3 on a decent headphone and soundcard and you should hear the difference. if not, the quality of the songs is ok for in a club.
Tough I still recommand re-ripping you're cd's to an Lossless audio format like FLAC, APE or Lossless WMA.
See wikipedia for more information on lossless.
I think it also has a lot to do with the software used for the compression. It certainly makes a difference. I can also remember a test where they had a single song at 3 different bit-rates and the original on cd. The had four audiophiles (people who have high demands for audio quality) listen to all the tracks and they had to make a list of wath sounded best. None of the testsubjects had the right list and only one had the original cd as the best souding (but the 128kbs song as the second best sounding). 1 or 2 had the 128kbs song as the best sounding. All this was tested on expensive reference systems...