[westinlennox]

iTunes (in your iPod settings) allows you to select a checkbox that says "Convert higher bitrate songs to _____kbps AAC".  If your music is already in that format, just select 256kbps and your iPod will free up some space, and you can keep your higher bitrate songs in your main library.  This is generally what I do.

 

[Doorbell]

I need to save some space on my iPhone, is there a huge quality loss converting 320 mp3 to 256 sac in the iPhone settings? 

 

[limpidglitch]

  I need to save some space on my iPhone, is there a huge quality loss converting 320 mp3 to 256 sac in the iPhone settings? 

 
Do you mean straight from mp3 to aac, without having a lossless master?

I've done similar things when I've been in a pinch, but I can't remember to have noticed any grave errors. It might be less destructive to transcode to a lower rate mp3 version rather than aac, as the codecs compress according to slightly different rules.  Better try it out on a few tracks first and see how it goes. 

 

[headdict]

  I need to save some space on my iPhone, is there a huge quality loss converting 320 mp3 to 256 sac in the iPhone settings? 

There certainly isn't a huge quality loss, but neither is there a huge space saving. You may want to give AAC True VBR a try, which adapts to the music's actual complexity and, depending on the latter, squeezes out some more (or, for jazz, less) space. You probably need some other software for this kind of compression, though. (I'm using XLD on mac.)