We're no audiophiles in my house, but both myself and my wife could tell which file was the uncompressed wav and which was the cd->mp3->ogg. An experiment with cd->ogg->mp3 which was not double blind (as the first test was) yeilded similar results: we could both tell. We were using Nine Inch Nails' The Downward Spiral album as the test case. The ogg was quality 6 or 7, and the mp3 was 128 bps.
It will depend on many factors on whether you notice a quality problem or not, such as the quality of the hardware you are playing on, the bit rate used, the program you used and its settings, the type etc... and even your own hearing.
All I can say is I have done ogg->mp3 transitions in the past (using transkode, the KDE frontend to transcode) and could not tell much difference between the original ogg and converted mp3. I cannot recall the settings but I believe the original oggs and transcoded mp3s where at a much higher bit rate than the 128 you used which might explain it (plus the fact I know my hearing is not great...)
Bottom line is all that matters is if you cannot tell a difference. So whilst others might tell you it WILL be terrible, you will only know if it matters to you if you try it out and see what you get.
Chris
