Inernal frames, internal frames, internal frames. Really.
I use Gregory, my girlfriend uses gregory, our buddy uses Arcteryx. I think Gregory's a step above Lowe's and Kelty (though Kelty comes mighty close) - I just found each thing on the Gregory packs I used to be just a little bit better thought out than on Kelty.
Arcteryx seems fab, but it's a price step up - maybe for the next pack, I'll get one.
People I trust like Osprey a lot too.
Gregory seems to be the standard "damn fine" pack, at least around here.
It may be worth it to pay an extra little bit and get the kind that has the detachable fanny pack built into the top part - that's nice for base-camp/wandering off sort of expeditions.
The rest of my load is a Sierra Designs tent (which I love) and an MSR Whisperlite Internationale camp stove (which I love), and a hand-pump water filter from, er, Katadyn. The PUR ones are great too.
If you want intensive, thorough gear research, check out The Complete Walker IV, by Colin Fletcher - great book.
-thi