Jaime,

Your intuition was right. What you really need to know is light intensity (not necessarily watts) per square inch of surface.

Watts per gal is an ok way to measure, but not great. Consider a 20L tank: (30"L x 12"W x 12"deep) vs. a 20H tank (24"L x 12"W x 16"deep).
If both have a single 175w MH, that's 8.75w/gal. Great, right?
The problem is that the 175w of light is getting progressively less intense as you go each inch lower into the water. So the w/gal metric does account for tank depth (which is good), but not very well for surface area (which is really the more important factor).

Say a 175w MH bulb puts out 10,000 lumens (this is the same as lux). That means the lumens per square inch for each tank are:
20L 10,000 / (30*12) = 27.78
20H 10,000 / (24*12) = 34.72

Real sunlight lumens per square foot is about 10,000. That's about 70 lumens per sq. in.

So the 20L ends up being only 40% of "real" sunlight values, while the 20H is 50% of "real" sunlight values.

That difference is small, but keep in mind that the surface area of the different tanks changes much more rapidly in reef-size tanks (65 vs 58, 70 vs. 75, 90 vs. 100, etc.). So this difference is much more pronounced as your tank configuration changes (even though the gallon capacities are similar).

One more piece of theory that (IMO) everyone should read once per year:
http://community-2.webtv.net/deflizard/doc/page8.html

Now, on Jaime's question:
120 tank probably 48x24x24 (?)
Surface area is 48*24 = 1152 sq. in.
Lumens depends on manufacturer; let's assume 22,000 lumens for the 400w MH, and also assume 3,000 lumens per 65w CF. Total = 31,000.
Light intensity = 31,000 lumens / 1152 sq. in. = 26.91 lux/sq.in.
26.91 / 70 = 38% of sunlight.

IMO, we should strive for 75% - 85%, so my opinion would be no, not enough light for SPS, clams, or Pacific host anemones. This would put your lumens target at around 60,000.

These are my opinions, and no one should think I'm saying this is the only way. Keep in mind that using a surface-area-based calculation, you still need to account for tank depth when you place the animals. Anyway....

Dean