Well with the tank being 48" long I see you have a few options. You could use a pair or 4 55/65W PC
bulbs. These bulbs are 24" long so you could put two side by side in the front and two side by side in the back of the canopy and your MH
bulbs in the middle. Another alternitive would be to use 96W bulbs that are 36" in length.
Before getting ballast from homedepot look at www.ahsupply.com
for their lighting kits. These kits include ballast, wire, connectors, reflectors and endcaps.
I am almost inclined to say start with 1 96W bulb or a pair of 55/65W bulbs and see how you like the look of the tank. If you want more blue then you could add another set of bulbs.