How many watts per gallon with metal halide lighting

I would like to set up a 55 to 100 gallon planted tank (I already have a
planted 20 gal) To maintain 3 wpg florescent or even power compact seems to
be a bit difficult, 8 4ft florescent tubes for 100 gal. So I think metal
halide may be a better choice. Does the 3 wpg gallon rule apply to metal
halide? Would one pendant over the center provide even light or do I need
two? Would 2 175-watt pendants be too much over a 55, how about a 100.

I would also consider something like in place of the pendants
http://www.drsfostersmith.com/produc...=20 04+113354

The tank would have a pressurized CO2 system.

Thanks
Scott