The road over the Andes from Mendoza to Santiago is incredible. I don't remember the name of the pass south of Aconcagua but I have never seen anything quite like it. Incredible switchbacks. The customs and immigration out of Argentina and into Chile were a major pain in the ass. It is a huge busy crossing and the Argys and Chileno's don't get along real well and are not very helpful, especially with the vehicle exportation/importation stuff. You can expect, as an American, to get hit with a $130 immigration fee when you enter Chile for the first time. Very important to carry that receipte as you head south. You will be crossing the border numerous times and you don't want to have to pay that fee again. Chile is not to be missed, especially Patagonia, but you could cut across to the Argentine Atlantic Coast to Comodoro Villa Darivia and then head south to Ushuaia from there and then come back up through Chile. Several ways to slice it. Enjoy!