When using HID's, I always attempt to have 50watt per sqft. <Optimal lighting in every grow publication I've read> Your 16 sq. ft in that case could certainly use 800w. Two 400w lights cost almost as much as one 600w. The 600's are disproportionately expensive for some reason.

I've always liked the 400w lights as you can hang them somewhat closer to your plant canopy, getting the most out of your lighting. Light deminishes very quickly, and every inch helps.

Is it nessary to follow a 50w/sqft rule? No. If going with less light will you have optimal growth and yield? No. It's a lession I've learned with somewhat disappointing yields in the past...B