Many factors dictate the pressure. We run 80 to 100 psi in our '00 because of the E350 springs and the age of the machine - it felt like the tail would drag on the ground if there was a dime in the roadway when we got ours 5ish years ago. Adding the bags gave us reasonable clearance and leveled the rig - this was one of the first additions.
Having bags won't increase your load capacity, but it helps with ride and leveling, and decreases the load on the springs.
Try running them at various pressures - you'll find a sweet spot over time. Try them at 10; try them at 100. Enjoy!