This will be an SPS/Clam tank primarily....
How about 1- 20k Halide ??
The Canopy will be at least a foot high w/fans...
Or 1-400 10k.?? w/ couple of Actinics....
Tank Dimensions: 36"L x 15"W x 20H......
Since your tank is tall, unless you are confining your sps and clams to the top, otherwise you will have to have 2 MH to get adequate coverage. If you are still in the planning stage, and can do without canopy, you might want to consider pendant type of MH. Tank clearance with these pendants should keep your tank cool enough provided your room has proper ventilation.
I think 2 250W pendant should do fine for you, maybe even the 175W types will suffice.
I have a tank with the same dimensions and I have found that 2 175 MH bulbs with fans in the canopy works great for clams and I have not had an increase in temperature. I do of course have central air in the house.
I have a 40gal 36" long tank with one 10k 250watt mh. My SPS and clams are doing great. I run a fan in the day and it heats up the tank about 2-3 degrees.
I would go with the 250's if your dealing w/ SPS and acros' they need the light especially for the depth of the 55 gal which directly relates to the wattage needed to properly maintain color and growth of these corals. If you get metal halides (which I highly recommend I use to have 4 110w VHO over my 75 but upgraded to 2 250watt 10,000ktemp MH and 2 110 watt actinic VHO's) trust me it's worth it (mine are in a hood with dual heavy duty fans).
