if your plants have just sprouted then as beta0701 says. 12-18inches is perfect... give them a day or more at a higher hight coz u dnt want them to burn.. after there used to the heat bring lamp dwn till its approx 18 inches above them...
use your fan to blow directly over the tops so they dont burn.. you might even get your lamp closer if you do this..
the more light youve got the more your plants will yield so if you had 200w then you would get double wot your gona get.. so on n so on.
Im growing about 9 plants in three pots under 150 HPS my light is about 7-10 inches away. I have a small oscillating fan a closet. If you want pics i could post... I dont even know why i would ask Who wouldnt want pics?!?