I have a single 250 hqi mh bulb in the center of my 40b and it does a pretty [language filter] good job of lighting the entire setup. Also the more MH's you add, even one, is going to raise that tank temp, do you want a chiller? or have one? I run 250 hqi mh with 2 t5 actinics and it pushes my temp. limits pretty hard, but looks really good.
Also you might be interested in building a canopy and doing custom retrofits, fun project.