Check out the Lowepro Orion AW - it is a belt pack that has a day pack which you can attach to turn the bag in to a rucksack.
I have one and the base holds - 2xD200s (both without grip, 1 with 35-70 f2.8 attached), 80-200 f2.8, 18-70mm, 50mm f1.8, 105mm VR Macro, Batteries, Memory Cards, Lee Filter Holder.