You will definately hear a difference between lossless and 320, particularly using the UM2's (which I also have used). BUT -- and this this a major but -- only if you also take the sound from a line out and use an external amp. If you send the signal through the earphone jack in the ipod, you are substantially compromising the benefits of the lossless format and quality earphones. even if you have a smaller ipod, it is still worth selectively synching your itunes library so that you have lossless files (albeit fewer of them at any given time) on your ipod.
if that makes sense, check out head-fi.org for detailed discussions of different headhone amp options, portable and desktop.
if that makes sense, check out head-fi.org for detailed discussions of different headhone amp options, portable and desktop.