Looks like the flight to Nashville might be worth looking into. Also might make more sense that way so I can hit Augusta to take up a GCAers kind offer of a hit at ACC en route to Savannah.
Scott - a few more random thoughts:
A Nashville/Memphis starting point to do a loop around the Southeast is a great decision.
I think Miami is one of the great US cities, but unless you really have a reason for going all the way down there, not sure if the drive is really worth it. Florida is a really long state without much visual diversity, and some of its cities (ie Orlando) are really not worth visiting.
If you cut out Miami or Dallas (blech again), then I would hit New Orleans on your southern loop. Even after the disaster, it's a culturally fascinating place.
If you must get a taste of Texas, then Austin or San Antonio are the better choices, but both are probably not the best to fly into.
I have driven your Denver to Vegas trip in the past and doing it again at the end of March. As you might realize the Salt Lake southward drive is just OK until you get to the Nevada border - the landscape and its rock formations are just stunning. Let me know if you want feedback on hotels or sights from Denver to Vegas. Just got back from a Vegas trip last week and the hotel prices are insanely cheap as the economy free falls.
Be prepared for the Los Angeles traffic. IMHO it's the worst in the country for LA is so large and spread out you could be stuck in it for "days". We live on the coast north of LA so feel free to drop me a note should you want to share a pint or two as you work your way up the west coast.
Finally, everyone should drive PCH (Pacific Coast Highway) towards Monterrey at least once for the views are world class. If you're worn out driving (and you may be by that time), please note the road is very curvy for a very long distance so you may want to consider an inland route.
The car rental choice is important given all the driving you have planned. A larger vehicle is recommended for comfort and safety. I have rented my share of smaller vehicles over the last year and the savings on the rental cost and fuel is not worth it. Although you are going to be hammered on charges for one way rentals. If you use the large rental chains (Hertz, Avis, etc.) then be sure to get something like the Ford Edge - it's a crossover between a car and an SUV.
Finally, purchase a GPS device (I prefer the Garmin brand). Most car rentals offer it as an option, but sometimes they sell out of cars equipped with GPS. You may want to buy one (less than $200 USD) when you get to the US for they are often country or region specific.