What strenght lights depends on tank depth. 30" or deeper to the bottom (top of sand, if you have sand) and you should think 400W. Below that, 250W unless you have a really shallow tank. For spacing, one light about every 2 feet works well for coverage (starting about one foot from the edge).
So, originally I had 3 400W pendants, now I have 4 250W in two lighting "units" (that also include t5 fluorescents) (my tank is 8' long, 30" deep to the sand). I switched because we are planning less sps so I don't need full-bright all the way to the bottom.
GK