AMS layover 8 hours

Well, you will have less than eight hours with trains and re entry at the airport. I'd plan on 4-5 hours.
For a short overview of the city I'd take the train from the airport, exit the station and follow Rick Steves walking tour. AMS is a very walkable city and it will be easy to find a restaurant as you cruise around. Another option would be to take a canal cruise.

I've never departed from the airport in Florence, but when we left Vienna a couple of years ago, bound for Chicago, with a plane change at AMS, the luggage had been checked all the way through from Vienna to Chicago and we didn't have to do anything at AMS. Same thing for a flight home from Rome with a plane change at AMS. And a flight home from Prague with a plane change at AMS. All were Delta flights. I believe that the general rule is that luggage is checked all the way through to final destination but this is not something you should rely on, as sometimes you might need to collect your luggage at the layover airport and re-check it. We once had a seven hour layover at AMS. I had learned that the airport has a tour desk offering guided tour options to travelers with long layovers. When we went to that desk to check out our choices, it was only 7 a.m., and no tours were going to be leaving for a couple of hours. The desk was staffed, though, and the employee assured us that leaving the airport and doing something on our own would be easy and cost-effective. He told us how to find the train station under the airport, gave instructions on what line to take and where to get off. The airport is only 15 KM from city center. Once there, he said, it would be easy to find ticket booths for a canal tour. We had to go through customs and immigration to leave the airport, but that didn't take too long. We timed our return so that we'd be back two hours before flight time, not knowing how long it would take to go through passport lines and security again. The canal tour was fun and with so much time before our next flight, we didn't feel stressed about cutting it close. If we'd tried to also work in a meal, I might have felt a little of that stress.

Taking a short trip into the city center is doable with 8 hours. This guide tells you everything you need to know:

http://tinyurl.com/h9m2kq3

Assuming you are on one ticket, you will get both boarding passes in Florence, and your luggage will be checked through to Atlanta. When you check in in FLR, make sure your bags are tagged ATL; if they are, you won't see them until you get to Atlanta.

If there is some glitch, and you either have to get boarding passes in Amsterdam or have to claim and recheck your bags there, just see how long this all takes before you make any plans about leaving the airport. In other words, while plan A will be to leave, plan B will be to stay. The Tripadvisor guide MIchael linked to in his post has suggestions for both scenarios.

You can store your hand luggage at one of many locker locations in the airport. Coming from Florence, you will probably land at the B or C piers and there is a locker location there. Returning, remember to go through security for those piers, collect your bags, then go through passport control for the flight to Atlanta. 8 hours should give you at least 4 hours in Amsterdam.

I'd say a leisury walk with a lunch or simple meal is the best way to use the time you've got. The map included in the link given above www.orangesmile.com/destinations/img/amsterdam_map_big.jpg is very useful. The city is at its best around the Brouwersgracht (on your right on leaving Centraal Station). Following the Prinsengracht counterclockwise, brings you along the Noorderkerk (North Church) and Westerkerk (West Church). Crossing to the Keizersgracht gives you the most monumental of the merchant houses along this canal. The end brings you at the river Amstel (from which the city got its name). Around here you find the Dutch branch of the Hermitage Museum. The semi-permanent exhibition »Look at us!« is a time-warp to the Golden Age of the city (http://portraitgalleryofthegoldenage.com/). Nearby you'll also find the Jewish Historical Museum with the impressive seventeenth-century Portuguese synagogue, a unique monument and in my humble opinion a more worthwhile remembrance of the Jewish history of the city than the Anne Frank Huis: http://jck.nl/en/location/portuguese-synagogue
Enjoy!