It also depends on what you want to keep in the tank. I have a 45g reef tank that is similar dimensions as yours, it's 36 long by 18 wide by 18 tall, and I run 2 VHO's (95watts each) and a 250watt DE metal halide. I think putting 2 x 250watt halides might be a bit of overkill, but that's my opinion.

If I had the room, I would have gone with a single ended 250watt halide as the smaller bulb and reflector of the DE limits how much the light spreads through the tank.