Here is another good driving direction/map site which seems to be more accurate that Mapquest,
MapPoint I found it referenced on another DIS post.
I've been the route from Memphis through Nashville and Chattanooga as annie2871 mentioned and it was longer than our other routes, but it is all interstate. Going from Memphis to WDW through Chattanooga is longer in distance and time than going through Birmingham; you go way north to get to Nashville then back south to get to Chattanooga.
If going the Birmingham route, the highway through Mississippi is just like an interstate (limited access and 70 MPH limit). Birmingham to Atlanta and Birmingham to Montgomery are interstate. If you go through Montgomery, the road (Hwy 231) from there down to I-10 has long stretches of 65 MPH limits, but you do get the towns. Even though interstate travel is the best way to go, there are times when accidents will shut down an interstate for several hours.
The routes I've been in order of preference (shorter time but it can definitely vary):
1--Memphis-Birmingham-Montgomery-Dothan-I10-WDW
2--Memphis-Birmingham-Columbus, GA-I-75-WDW
3--Memphis-Birmingham-Atlanta-WDW
4--Memphis-Chattanooga-WDW