What Type of Backpack is best?

OK, I've leaving to go on a 9-12 month trip around the world in about 6
months or whenever I get enough dinero. I got money for Christmas and
am going to buy a backpack but I don't know which one to get. I'm going
to be on a tight budget for the trip so I'm not gonna buy many
souvenirs and whatnot and will survive on 2-3 outfits. But I'm probably
gonna need atleast for a while a winter jacket (I think). A Couple
questions:

Internal Frame vs External Frame?
What Size?
Are Layers better than a winter jacket? (will that save space?)
Brand names? Model names?

Thanks everbody!

Bryan Dunn