Custom iPhone "New Text Message" tone?

I and my friends have have iPhones. I am the only one with Gen one because Karma/Fate/and anything else I can blames, hates me.
Anyway, we want to know if we can add our own custom text message alert sound. Everytime one of us gets a message we all check our phones. And yes we know we can use diffrant tones built in but they all suck.