Deep South, as far as I've always reckoned, consists of Alabama, Louisiana, Mississippi and south Georgia. Tennessee, North Carolina, South Carolina and Virginia are all considered Old South. Arkansas and Kentucky are also part of the South Florida, as we all know, is part of New England.
Some historians consider the Deep South to extend far, far north of that, however, stretching up the VA coastline... but mysteriously excluding the Appalachians.
I don't consider NC part of the Deep South. I don't even really consider SC to be Deep South, nor northern GA. Southern GA and anything with a Gulf Coast is Deep South for me.
EDIT: Except Texas. Texas is Southwest.