There’s a definite difference between trying to get to a destination quickly and having time to stop and meander. However long you thing it will take you, I recommend you add a few days to that. See, everybody starts out all excited, because, “woohoo it’s a road trip”, but midway through the second day they realize what a road trip actually means. I’ve done some 2 and 3 day trips and we really pushed through, but it was rough. Not being super pressed for time, where you can take an afternoon or a day off for something other than driving, that can really help. That, or Ripped Fuel, Hydroxycut, No Doz, Monster & Red Bull and cigarettes.
Burn lots of CDs. If you have an MP3 player hookup, work on adding lots more music and playlists, and bring a car charger. And burn lots of CDs too.
Spare tire, jack, at least one (maybe even 4 if you’re high speed) Fix-A-Flats, roadside emergency kit, some random clothes, and a sleeping bag. Because you never know.
I’ve done 40 and 20. 20, I think, is preferable to I-10 at least going through Louisiana. Watch out for troopers tho; I always see a lot of them at night and they won’t take no guff. Mississippi’s a fine state and very pretty. I don’t care too much for Jackson but I found the highway layout around there to be sensible enough. Meridian is a nice enough little town. It may be a little bit out of the way but I (almost) recommend the Natchez Trace Parkway. It’s a nice scenic route, and fun to drive if your car’s a 5star on handling. But I once got (inadvertently) stuck on it and drive all 400+ damn miles of it, which spat us out somewhere in Tennessee.
There’s always traffic and construction in Atlanta. Columbia, South Carolina isn’t as great as they advertise. North Carolina is pretty, and they used to have some of the best roads in the country, but after enough hurricanes they have some of the worst, so watch out for that.
Which reminds me. I-40 doesn’t go through Dallas; it New Mexico, Amarillo, Oklahoma City, Little Rock (Arkansas is very flat, it is quite a change from Tennessee), and into North Carolina through the Great Smokey Mountains. GoogleMaps sez that is shorter than going through Dallas. Tennessee, by the way, is a real morale buster, because you are going through it the long, horizontal ways, and it best it will take up most of your day, but can easily last two.