Well, yes and no, you shouldn't really translate it. It's more a way of speech, rather than being an actual word. That, and Dattebayo just sounds way more cooler than adding "Believe it" after each of your sentences. I mean, they did it recently with the dubbing of the new Star Ocean game, some little kid there constantly says "yo" in the Japanese dub, but they actually translated that to the English dub...now the constantly says "'kay". I mean, stuff like that isn't supposed to be translated to English.
There is no actual translation for the word 'dattebayo' (
http://www.freedict.com/onldict/onldict.php - Tick exact off, and enter 'dattebayo'), it's just something Naruto made op to make him stand out more, the English dub just translated it to "believe it" because if they didn't, they would get some lip syncing issues.