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

I took an Arc'Teryx Khamsin 52 (4400 cu/i) on my last 3 month trip, and it worked out great ... very comfortable. Probably could easily do 9 months with it.