metal halide questions, how many watts?

[let_it_grow]

I'm gonna be ordering a metal halide bulb and ballast but when I was liking into then I didnt realize that there was such a wide range in wattage. I'm growing Cyms and Dens and some other high light plants and I have a 9" x 8 1/2" north window that gets a decent amount of light so its supplemental light for my little guys. I have confused myself by looking into it, there's like a million different kinds, I thought 250 watt would be enough but now I'm not sure. Help