Thanks guys. From what everyone is saying, it sounds like we may need to cut some driving hours down. The trouble is that means choosing between Nashville and New Orleans., which is a tough one...

We did consider doing a one way drive, but it costs so much more in car hire fees, and also plane flights that we decided in the end to do a round trip and do the east side of the south another time.

We will only have a maximum of 3 weeks (as work will not allow us longer), so will have to fit as much as we can in, but at the same time we don't want to do so much driving that we miss all the good stuff.
We definitely want to do some of the sticks as well as the main cities and tourist points.

We have a clash of interests because I really want to go to Tennessee and Texas, and Sam wants to drive the Mississippi down to New Orleans....

So much to see, so little time.

We have a while to bash out the details though, so everyone's thoughts are greatly appreciated.