If you haven't already found them, there are lots of excellent articles on lightening your load and on gear selection listed in the left-hand column of http://www.backpacking.net/, the home page of this site. They helped me cut my total pack weight for 7 days by more than half (down to 22-23 lbs. total pack weight for a 7-day trip), without sacrificing either comfort (which, as an old lady, I need a lot of!) or safety.

Another good site for gear selection is Mark Verber's website. Lots of ideas for gear, from the latest technology to ultra-low-budget alternatives. Also tons of links to reviews and other sites. Especially read what he has to say about lightweight (not UL) packs.

As mentioned already, you should wait to get your pack until you have most of your other gear. You don't want a pack that's too big or too small. If the gear doesn't fill the pack, it will be out of balance (and you'll be tempted to throw in stuff you don't need). If the pack is too small and you have to tie stuff on the outside, the pack will also be out of balance (and you're liable to lose some of the tied-on stuff). In the meantime, beg, borrow or rent.

You want a pack that comfortably supports the load you're going to carry, but you don't want a heavy 5-6 lb. bomber pack, either. Your legs and feet will thank you for a lighter load, and you'll be able to hike farther!

I'm more familiar with the PCT than the AT, since the PCT goes by about 20 minutes' drive from my house. I know that most PCT hikers plan to resupply at least once weekly if not more often, so they are not carrying huge weights or volumes. In fact, most of the successful ones try to keep their base weight (everything but food, water, fuel) at 10-12 lbs. and their total pack weight not over 25 lbs. It might get more than that in the drier regions where they have to carry more water and in the Sierra where they have to carry 2.5 lbs. of bear canister. I do know that most of the people that start out with "standard" heavy gear and 40 lb. packs either end up ditching it in favor of lighter gear or quit the trail. If your eventual goal is the AT, you might want to check out some trail journals on trailjournals.com and postholer.com. A number of the journalers include their gear lists.

When I researched my lightweight backpack 5 years ago, I picked about 5 that interested me. My criteria were that the pack be 3 pounds or less and be able to carry 35 lbs. comfortably, not that I wanted to carry that much. Oh, yes, it also had to fit me! Load lifters were a requirement because my shoulders are very pressure sensitive, which left out most of the lighter packs like those from Gossamer Gear.

I came up with a list of five which I prioritized in order of weight (not necessarily a good idea, IMHO. I then went downtown to REI and tried on almost every pack in the store. None of them met my weight criterion, but at least I got a good idea of how a well-fitting internal frame pack should feel on me (my old pack was a 5-lb. external frame). I then started mail-ordering, assuming that I would have to pay return postage on several before I got what I wanted. The first pack on my list, as best I can remember, was the Six Moon Designs Comet with the "optional" (IMHO mandatory) stays. (This pack is since discontinued, but its bigger brothers the Starlight and Traveler are still made.) The others were the Granite Gear Vapor Trail, the Granite Gear Nimbus Ozone and the ULA-Equipment Catalyst. I'm sure there was another one, but I've forgotten by now. I got the Six Moon Designs Comet first, loaded it up with my gear plus the weight/volume equivalent of a week's food, and hiked around the house with it for a couple of hours (most boring hike I've ever done, but at it gave the pack a fair trial while keeping it clean so I could return it). It felt as though it had been custom-made just for me, and still does after 5 years!

Sorry about the long post, after you've already been swamped with references!