Hi Yoda,
I have hired a car in one state and dropped off in another many times, but this MUST be agreed first.
The second thing to consider is where you hire the car from, and I mean by this that it will be cheaper to hire in the US, but most likely will not include much in the way of insurance, as Americans can transfer their existing car insurance onto a rental car, where we can't, so I have always used Dollar, but booked it on a UK website, if this makes sense?
Make sure you get EVERYTHING covered as when things go wrong out there it can get very expensive!
Have you looked into the time and distance involved in driving from Atlanta to Orlando?
I am convinced it will be cheaper to fly?
I used to either fly with Virgin directly to Orlando or fly to Charlotte with US Airlines and then get a flight to Orlando.
More than happy to help in anyway if you need/would like further information.
What ever you do have a great holiday.
Nick