A lot will depend on the actual length of trips you have in mind. For up to three or four days, 4,000 cubic inches or less is perfectly adequate. That would mean something like the Osprey Crescent 75 (4,500 cubic inches, $349; www.ospreypacks.com) or even the new Osprey Aether 60 (3,800 cubic inches, $199). I've used the Aether 60 on a couple of weekend trips, and have found it more than adequate provided I pack rationally. The Dana Design Glacier ($299; www.danadesign.com), on the other hand, could handle a week's worth of equipment and supplies. It has about 5,000 cubic inches of capacity, depending on the size you go for, with a suspension capable of hefting loads of 50 pounds or more.
You might also take a look at packs from Gregory and Arc'Teryx. In the Gregory line, the Shasta (about 5,000 cubic inches, $249; www.gregorypacks.com) is a superb pack for the money, with great suspension and excellent bag design. The Arc'Teryx Bora 65 (up to 4,600 cubic inches, $325; www.arcteryx.com) makes great use of the latest pack and harness technology to yield a versatile, go-anywhere pack. Finally, Mountainsmith has carved a strong niche for itself with its Auspex ($260), a very light backpack that still carries more than 4,000 cubic inches of gearpacked properly, that should be plenty for up to four or five days.