Out of interest, I compared spectrograms of a few re-compressed (lossy to lossy) MP3 files. The source was a two-second clip of a 320 kbps CBR MP3 (WAV of it here - the original source of the MP3 was an online music store). The other points of comparison were a few LAME re-compressed version of that clip: V0 (~245 kbps), V2 (~190), and V6 (~115).
 
The spectrograms, from top to bottom: original clip (highest peaks = ~16.3 kHz), V6, V2, and V0. On the right of each image is the difference vs. the original MP3 clip (darker shade = more difference).

  
  
  
 
I also compared a V0-compressed version of that clip vs. a V0-compressed version of that V0 version (so 3rd generation). The difference is below.

Wow your guys knowledge is truely awsome, just reripped all my music to losslees using iTunes was easy and painless but wow does it ever eat up disc space Thank you all