Sorry, didn't see this one.
The main place that is a bit of a nuisance regarding the trip is Bariloche/Santiago, as you have to either go back from Peru/Bolivia to Santiago. Or is you visit Bolivia/Peru first head south and do a return trip Bariloche basically from Santiago. But well, I guess you know that already
Maybe a point is the holidays of Argentinians which is from half December to January (maybe early February). If you want to avoid most of the crowds I would do it at the end of your trip as Bariloche weather is still agreeable in March (your last month) and most of the local crowds have gone to work again.
In that case, an option is (and I add some points of interest which you haven't mentioned):
Rio - Pantanal - Iguazu Falls - Buenos Aires (maybe side trip to Colonia in Uruguay) - Salta and northwest of Argentina - to Tupiza - taking a 3-4 day trip from Tupiza to the lakes and salar de Uyuni ending in Uyuni - La Paz - Lake Titicaca - Cuzco and Machu Picchu - Arequipa - Arica - San Pedro de Atacam and surroundings - Santiago - crossing the Andes to Mendoza - south to Bariloche - cross to Chile from there - north again to Santiago.
I think first month Rio to Bolivia, second month Bolivia/Peru and third month Chile and Bariloche is possible without rushing it too much.
On a side note: if you are able, check if it's possible to end in Buenos Aires. Somehow it's easier with the loop then.
Have fun!
Michael
[ Edit: Edited on 01-Aug-2009, at 08:35 by Utrecht ]