so u have a choice..... either mess up your game and get a mew or not mess up your game and not have mew [just get a mewtwo]
you can also get mew on emerald. you just have to have to Old Sea Chart, which will take you to World's End Island. once there, you'll have to play hide and seek with mew uintil you corner it. then you can battle it (note: it's at lvl. 30). then you can trade it to your other games.