i use this program :
it will set the gain of the mp3 without re encoding them like other apps do and only takes a few seconds per song. and its totally reversible.
it has the option to do an album gain also, so it scans all the songs from the same CD and adjusts them accordingly so the softer ones stay softer and the louder louder.
the default peak setting in the app is 89 but i think this is far to low. most mp3's i get are ether 95, 98 or 100.
i have mine set to 95 but this is just a matter of preference..
hope this helps