Hi -
A typical setup for your tank would probably have 2 - 175W MH setups, with a 48" actinic or two, depending how much supplementation you need. There's lots of research and planning ahead...
I would take a look at anyone's pictures here that lists a website in their posts or profiles, and maybe use the "search" function in the DIY section for lighting, hoods, metal halide, etc. You'll likely find examples of people with tanks similar to yours, and the solutions they came up with.
You'll find that a single 175W won't quite cover a four-foot tank (your 72 is 4 feet?) The spread for a single MH is usually 2 feet, maybe 3.
I'm sure others with more experience will chime in now...
