If you do plant Milkweed (any of the Asclepias mentioned above) remember that it is not only a nectar plant for butterflies, it is also a host plant for the monarch larvae, so all the leaves will get eaten up at some point. Since it's not awfully attractive after this happens, it's a good idea to interplant it with something else, like Gaillardias (which are also nectar plants). The clumps of Gaillardia will hide the bare stems of the Milkweed until they manage to leaf out again.
Buddleia is a great butterfly plant and smells good to humans, too. There are several different colors available that don't set seeds.
