The places you picked are pretty good for giving you a global idea of how diverse Brazil is... you'll see a bit of everything
Brasilia is cool for the arquitecture but the city is just boring. Curitiba is a lovely city but it doesn't have anything absolutely amazing that you just have to see, and Sao Paulo is just one fucking enormous city, which I love of course, but doesn't vary much from any enormous city. So these are are places you can visit for just 2 or 3 days, so you save some more time for the other places that demand more time.
In Sao Paulo I recomend you going for a walk in Paulista Avenue and nearby streets and have lunch at A Mineira while at it, or Bolinha's restaurant for the best fucking feijoada you will ever eat. Also in Paulista there is MASP, an art museumm and a bunch of other cool things. The Ibirapuera Park is also a cool place to go, there is always something happening there and there are lots of museums in it.
Stay at least a whole week in Rio. Do all the typical tourist stuff because it's just beautiful. Or not... spend all the time by the beaches if you feel like it... Rio just has such a cool atmosphere you want to be there and hang out. There are lots of bars with live music, which I know you're interested in. Just avoid the yellow and the red lines and DONT GO IN ANY FAVELAS, even if you meet some cool guys who invite you in to smoke some pot.
I've been to Salvador once and it was for work, but even then it was pretty cool. The food is amazing and the beaches are beautiful. There is also a lot of interesting stuff happening there musicaly speaking. Go to Pelourinho, go to Farol da Barra, go to Mercado Municipal, go see Timbalada.
I've never been to Fortaleza, but my sister has and I've heard many good things about it. Beautiful beaches and etc...
I think you should go to Ouro Preto in Minas Gerais. It's a historic city from the times when Brazil was huge on sending gold to Portugal. Just google it for images and see if that kind of stuff interests you. It's also an university city so there is lots of stuff to do there at night with the younger croud, make some friends and etcetc
Foz do Iguacu is AMAZING, i went there and it blew my mind, it's just too beautiful. Go to the Brazilian side of it, of course, where you see the water from below, and you go so near you get completely soaked. It's awesome, you'll love it.
I've never been to Manaus, I'd love to go.
There is a lot of other places that I could recomend, even some places that I haven't been yet and would like to go, but I think the places you picked are more than enough for 6 weeks. It'll leave you wishing you could stay for longer
Anyway, find the places you like the most and then come back again to know them better
http://www.youtube.com/watch?v=-OPVl6kiN8I