I agree with Bill and phat. Get the rest of your gear list where you want it, measure it (those plastic storage bins are good for approximating, since the bin's tag usually shows how many liters the bin contains, and there are approximately 60 cubic inches in 1 liter) and buy your pack last. Be sure to include the equivalent weight and bulk of food, fuel and water for the maximum time you expect to go out. Load up the new pack to be sure everything fits inside, and then "hike" around the house with it for several hours. If you're getting the pack at a store, take all your gear with you when you go and do the "hike" in the store (do a lot of going up and down stairs if you can).

Be sure that your total pack weight (including pack) does not exceed the maximum carrying capacity for the pack in question (usually found on the manufacturer's web site). In fact, total pack weight should be at least five pounds under that maximum. A 10 pound margin would be better if you're going to be in desert areas (that's 1.25 gallons of water).

My own experience with external vs. internal frame packs is that the external frame has a life of its own, often zigging when I zag. I've had several falls as a result. The internal frame, on the other hand, moves with me and very rarely pulls me off balance. There was the time when I didn't duck low enough when going under a leaning tree, and ended up sitting down abruptly (just missing my dog) when the pack caught. Except for that episode, I haven't had the balance problems I used to have with the external frame.