The Serpent mound is earlier and part of the Adena/Hopewell complex. Slightly different mound construction, not quite as organized and earlier than the Mississippians.
Cahokia is probably the biggest Mississippian city, along with Etowah and Moundville in Alabama.
Are the Ocmulgee Mounds in Macon, Ga also Mississippean? Very similar design and layout.