well, it depends. are you going to hang them from the ceiling? if so, the one light will be fine. you can just push it out of the way when you want to get in the tank.
do you have 1 center brace or two? with just one brace, i think you might have trouble lighting it with halides, unless you can find a 36" fixture that has 2 lamps in it.
if you have 2 center braces across the top of the tank you can use a 6' fixture, and you still may have a problem with 2 x 36" fixtures.
unless you go all T5. then you can have any amount of braces.