Depends on how big your plants are gonna be and how much room you have. I usually figure on at least 1.5 square feet per plant....about 15" by 15." More room is better, but I think you can get 6 plants into a 3' x 3' space. For that area, a 400 will be fine. HPS lights, unlike cfls, get more efficient as they get larger. A 400w bulb will put out 50,000 lumens, which is 125 lumens per watt, and about 5500 lumens/sq. ft. in a 3' by 3' grow area. You'll do fine with that, no doubt.

But if you're going to have bigger plants (over 4') or more than six plants, definitely go for more light. A 600w plant can handle a 4' x 4' area, and that's enough for 8-10 plants.