I can't really recommend any one pack-that will depend on what fits you and your budget. But, most manufacturers, such as Granite Gear, will have a chart on their site that will show the recommended load limit for their packs. I would start with that.

You may see some recommendations here, but be aware that some ultralight packs are not designed for 40 or even 30 lbs. They may have very lightly padded straps and a belt that is little more than something to keep the pack from shifting around, as opposed to transferring any weight to your hips.

If you can get to a store like REI or a local outdoor store that sells quality gear-not Wal-Mart or equivalent, they should have a variety of good packs, plus weighted bags to load up the pack so you can walk around the store and see what 40 lbs feels like.

Whatever you do, make sure the pack fits right. That is more important than brand or minor differences in pack weight. If the store has someone who knows what they are doing, they should be able to help you get fitted properly.