There's really no difference. "Over there" is used a little more for things that are close to the speaker. For example: "Go stand over there so we can toss this baseball." However, I've heard it used for large distances as well. For example: "Do people in Australia really say 'G'day' to mean 'Hello'?" "Yeah, that's just how they do it over there." Whether you use one over the other depends on prosody more than anything else.