The most important part of a backpack is fit, fit, fit. Backpack fit is almost as individual as shoe fit. You can't just order something and assume it will work for you. The pack has to fit your gear (don't get it until you have all your gear, or at least all the major pieces) and has to fit you with your gear in it. Once you get the pack home, load it up with your gear, a couple quarts of water and the equivalent in weight/bulk of a week's food and "hike" around the house with it for several hours--do this while you can still return the pack.

Have you read the articles on gear selection on the home page of this site?