I have one to top all of you all. I have just departed FTY on the west side of Atlanta at 3000 ft heading 200. I get a traffic advisory from ATL that there is traffic 10 o'clock same alitude West bound. At the same time I look in that general direction and see an airplane. I acknowledge the alert, tell him I have him sight. Well, at the same time, my Capt. catched a glimpse of him as well, and the whole time this other airplane is starting to get bigger and he is moving. Please keep in mind this is all in the span of seconds. Ok, So Capt sees him, next thing you know, this airplane is HUGE in the winshield, ATC(in a very calm voice tell us, "Do what you have to do...") So we put the airplane in to about 30 degree dive, Capt flying, I am looking outside, The other guy, puts his airplane into climbing right turn as well. The Skywatch in our airplane says that the other airplane, looked like a Meridian, was +200 ft above us, and we were in a dive. Come to find out, that guy was heading west at the wrong altitude, and the ATL sector he was under hadn't coordinated with the guy we were talking to.