From the way you pose your question, I suspect you will end up like me - with enough old bags to open a Backpack Museum. It's OK - they can be recycled into many different uses.<br><br>In general, I like a large waist bag for daily carry. Others probably go for a pack with about 2000 cu in.<br><br>For longer trips, or day trips with more gear, look for somethin in about 3500-4000 cu in.<br><br>For the week long trip, you wnat at least 4500-5500 cu in, and pay a lot of attention to the suspension.<br><br>There are numerous brands out there, and quite a few of them are very good. Fit and engineering become more critical with the larger bags which will carry more weight. Buy what works for you.<br><br>Don't be misled by the gadgetry that festooons many models - extra straps, pockets, and so on. These just add weight and complexity. Remember that you can add and subtract fixtures, since you are dealing mostly with easily alterable fabric.<br><br>All in all, I prefer a top loading bag, internal frame pack in anything of any size