You'll probably be able to grow plants from at least zone 6b, and perhaps 7a and 7b as well. Hardiness is usually expressed as a "range", such as "hardy to zone 5." Such plants will grow nicely in warmer areas, and there are usually different varieties of a species that can take more cold or heat. There really won't be all that many limits on what you can grow.
Depending on how much effort you want to put into it, you might even be able to succeed with plants from much warmer zones. Giving the plant what it requires is what it amounts to. If a particular plant can't handle the winters in your area, you might be able to grow it in a container and bring it indoors during the cold season.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams