Halogen, Sodium, and Metal Halide are different animals. Assuming you are referring to a Metal Halide 400 watt fixture, You will need to use double pole breakers, double pole switches to interrupt both sides of the circuit, and a ground wire. Are you using Romex or conduit?
If you have sodium fixtures, just go ahead and scrap them and look for something better. You will hate the orange light the moment you turn them on.
If this is in a Residence, lighting over 125v is not allowed by NEC. If it is a detached shop or garage, then the 240v would be OK, as the code is specific to a residence on the voltage limitation for lighting.
They are 400 watt bulbs, the fixture actually draws about 480 watts, so 80 watts is wasted in heat at the ballast, and it is 4 amps at 120v and 2 amps at 240v.
Lighting is generally considered a continuous load, which the NEC limits to 80% of a circuits capacity. Thus, on a 20 amp circuit, 16 amps would be max, and on a 15 amp circuit, 12 amps would be the max.
NEC defines a continuous load as anything that is expected to be on and drawing its full load for more than 3 hrs. Some things, such as heating devices, are specifically defined by the code as continuous, others, such as lighting are left to the imagination of the installed, based on expected use.
Charles