Quoting Sander
You can totally skip Atlanta. Nothing worth seeing whatsoever (which can't be found in any other American city as well), other than lots of streets named "peachtree something". *g*
I'd recommend Philalphia if you want another American city in the east to visit. Better yet, drop Miami too and spend some days in San Francisco instead. 
I almost always say "listen to sander" as I almost always agree with him
but not this time.
I do not know anything about atlanta other than braves baseball, coca-cola, and that giant aquarium so I can't really say.
but if you are looking for a good party scene....DO NOT SKIP MIAMI! infact if you have the time I would stay a few extra days there. Miami is glorious. Beautiful women everywhere, beautiful beaches, so much culture, great food, clubs everywhere. Go to the beach on miami beach where it's less crowded and party on south beach.... It's awesome.
take a bus to philly (my hometown) from new york (it's only an hour and a half) for a day then get on a bus and go to the D.C./Baltimore area (another two hours away) the inner harbour at baltimore is pretty cool plus their aquarium is top notch. and then D.C. has a great party scene. then fly to miami.