"Buddha" is not a person, it is a title that means "the enlightened", just like Christ is a title that has been bestowed upon Jesus. Throughut history there have indeed lived many Buddhas and many others are still alive today. According to some traditions every person is a Buddha, some just don't know about it.

As for the person you are referring to, you probably mean Siddhārtha Gautama. He has indeed lived some time between 600 and 400 BCE (the exact dates may vary according to different sources).