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.