Check out articles on the home page of this site, left hand column for information on selecting packs--there's one on bad back issues, too!

I personally need an internal frame--external frames tend to zig when I zag, causing falls, while internal frames move with me. Do note that pack fit is almost as individual as shoe fit, so what works for one of us may not work for you. For example, my hips are completely incompatible with Osprey hip belts, so for me that brand is not an option, while a number of folks on this forum think Osprey packs are wonderful. My suggestion is to pack up what you want to put in the pack and take it with you to the local stores to try out what's available. The pack needs to fit you, fit your gear, and, above all, be comfortable for you to carry with your gear inside.