I'm a pretty avid backpacker and have had many packs through the years. I started out with surplus army packs, but the civilian internal frames are much better for comfort. My current pack is an Osprey Aether 85. 85 stand for 85 liters. It's a great pack, very comfortable and light for it's size. It isn't a cheap pack, but I've found good quality rarely is. REI carries this model as well as larger and smaller ones. Good luck, maybe I'll see you on the trails somewhere.
Dan