tank depth

I currently have a 75 gallon tank (48" x 18" x 18") with a Current Outer Orbit HQI Lighting system (Model #1042). I'm thinking about upgrading to a 120 gallon tank (48" x 24" x 24"), which means I'll have about 4.6 watt's per gallon. The MH's in the system are 150 watt bulbs. My current...