Youll have to carry a pack that size in order to bring all the insect repellent youll needha, ha, ha!
But, yeah, several packs should be right for you. These days I tend to regard the Gregory Palisade ($299; www.gregorypacks.com) as the classic big backpacking pack. It has an excellent suspension that holds up under big loads and transfers the weight well to its firm, contoured hipbelt. In the medium size it holds 5,100 cubic inches. Access to gear is fairly easy through a top port, front zip, and bottom sleeping-bag compartment. And theres lots of room for odds and ends, plus you can choose between using a hydration bladder or slipping water bottles into several easily accessible pockets.
I also like the Osprey Crescent 85 (www.ospreypacks.com), although its a bit pricier at $380. Osprey suspensions are just a cut above those on Gregory packs, with perhaps slightly better load transfer and a more comfortable hipbelt. But its not a big gap. Osprey also adds some nifty features such as a waterproof compartment for wet gear (or to keep gear dry). Capacity is the same as the Palisade.
For a real bargain, look no further than the Lowe Alpine Contour 80+15 ($239; www.lowealpine.com). It has a simple but effective suspension with curved aluminum stays to get the load to your hips, lots of places for small items and bigger things such as sleeping bags, and plenty of space (4,900 cubic inches, plus 900 more if the bag top is extended).
If you can, try a few packs on with 25 to 30 pounds in them. Packs are a little like shoes in that fit matters, and not all packs fit all people. Plus think about how you like to pack a pack, and try to imagine yourself using the pack during the day. What makes the most sense, in terms of pack layout and where daily/nightly items are stashed? And of course, what seems to hold the load best.
Check out this years more than 400 must-have gear items, including a comprehensive backpacks section, in the 2006 Buyers Guide.