Just to give you an idea of the timescale for BA to Rio:
Buenos Aires to Puerto Iguazu by bus will take approx 20hrs+. I'd recommend opting for the coche cama class. It's comfortable for the overnight journey. Via Bariloche are a good company to travel with
Give yourself a day or two in Puerto Iguazu to see the falls. I'd recommend staying at Hostel Inn - they will organise to transport you to the falls.
From the bus station at Puerto Iguazu you will need to take a local bus to Foz do Iguacu on the Brazilian side (a short trip with a stop at immigration). At the station in Foz you can book your trip to Rio. This journey will also take approx 20+hrs. I'd recommend travelling with Pluma. Bus travel in Brazil isn't up to the standard of Argentina but it's still comfortable.
That will give you a few days in Rio
Not sure if you are hostelling or not - if you are check out Mellow Yellow - a great hostel not far from Copacobana and Ipanema beaches.
Hope that helps!
[ Edit: Edited on Sep 20, 2007, at 1:33 AM by samsara2 ]