iTunes is the way to go. Open the iTunes preferences, and go to the Importing tab. Select the MP3 encoder, and choose the "Custom" setting. You can enter whatever bitrate you want there. Then close the preferences, select whatever songs you want to convert in the main iTunes window, and select Advanced menu -> Convert Selection to MP3.
I imagine one of the Toast products (not sure which) can accomplish the same task, too, but I doubt there'd be any real advantages over iTunes.
Well: If you've already _got_ those songs in iTunes, iTunes might _not_ be the way to go, because it can be a drag to _find_ the right versions afterwards. (You'll have two copies of each song after reimporting them at a lower bitrate.) Smartlists might do the trick (dunno), but that doesn't work for deleting the songs IIRC, since you've gotta select them in 'Library' in order to actually trash them. (Unless I'm just wrong. Please correct me if I am...)