192 VBR for most pop/rock stuff with no dynamic range anyway.
256 VBR for anything that demands absolute fidelity.
256 for anything over 10 mins or so because iTunes is absolutely worthless for dealing with VBR files much over that length (unresponsiveness skipping through the track, total sound cut off at places till it finds it's place again, this is PATHETIC!!! and some people with iPODs have a similar issues with long, DJ or live sets).
Just talking about mp3s, Not AACs yet.
I can't use them till I get at least an AGP graphics G4 because iTunes 4 is OS X only and my 'supported' mac isn't 'supported' by any of the OS X versions of the software I currently run fine on my G3 under OS 9.