dont get me wrong, 3x150 is good lighting, but when your talking about a 72inch fixture generally its going over a larger and consequently DEEPER tank.
150w seem to do best when tank is under 20 inch or so.
Deeper like 25 inch and 28inch tanks seem to do best with 250s in my opinion. If she shoe fits then go as low as possible. Less heat, less powerbill and cheaper replacement bulbs and ect. I always go overkill, but I like just never worrying about what or where I ever put something.
I cant remember if I can post a link or not to this site so email me if you like. Or If you have yahoo messanger my id is the same.