The 36" size seems to be the most difficult for metal halide applications. I finally opted for 2x 250w DE for my 36" x 24" x 20" (75G), with the ability to dim the 250w bulbs with an electronic ballast if necessary.
None of the high end lamp manufacturers give a good option for the 36" size, as one DE lamp over 36" isn't going to be enough coverage in the small reflectors that Giesemann type lamps provide.
A single 400w or 2x 250w is what has been recommended to me. I would think the 2x 250w would give better overall coverage, but obviously a 400w would have the best punching power in the center.
The question that you need to ask yourself is what do you want the lighting system to look like? If you don't want a canopy, then the Sunlight supply type reflectors that are necessary for a single 400w bulb and the required coverage will look gawd awful. There are pendants that would work well, but then getting supplemental lighting can be difficult and also an eye sore. I also wouldn't mess with the discount brands like what you posted.
ReefBrite makes a decent looking pendant that you can add their LED strips to, but they are insanely expensive for such an industrial looking fixture.
-Mark
