pocketnico saidTo me "how do you do?" sounds more Southern than old fashioned or perhaps a bit of both. Or maybe that expression has survived in the South longer than it has in the rest of the US.
I think some of the "How do you do?" survival in the South has to do with social formality, and to say the British influence in the South is trivial would be grossly misrepresented. I think that is why states like Virginia and South Carolina have so many customs --- especially with the 'well-to-do' --- in common with the British 'well-to-do' and aristocracy... at least in the recent past.
When I met one of my roommates for the first time in college I said, "How do you do?," and he said his first thought about me was "old Southern money." That is just how I talk in general, but I didn't really say it to come across as a snob. Had I say "How y'all doin'?," I probably would have been more endearing.