Some questions about travel to brazil and argentina?

First of all does anyone know when Carnival takes place in 2012? Also, I'm thinking of starting my adventure in Manaus but hope to be in Salvador by Carnival. I want to travel by land and water through Brazil and go from Manaus to Salvador and then to Belo Horizonte which is where some good friends live. From there I hope to journey on to Rio or Sao Paolo and then to Iguassu Falls and on to Buenos Aires, possibly Bariloche, and eventually Ushaia, Argentina all by land as well. I already looked into flights so what I do need help with is looking into bus fares or other fares (ferry or riverboat?) to make this adventure a reality. I'm a 31 y.o. female that is strongly considering this journey alone. How safe is it? I hear Rio is dangerous but many also recommend that a trip to Brazil is incomplete without a stay in Rio. I'm thinking of staying in hostels or if that's not ideal budget hotels. I want to go on an adventure but hope I make it back home too!