I think the best equivalent would be 'The dream is peace' . This is because of one little word... 'the'. If the exhibition title was 'Peace is a dream' then your other equivalent - Peace is nothing but a dream' could apply, but you can't be sure what was in the mind of the person who wrote it. Maybe, 'A dream of peace'. - Note: in this example you can use the preposition 'of'. These prepositions are tricky! As I write this I am asking myself, "does this sound correct?" to me, from New Zealand, it does.