400 Watt HPS light system

So here's the deal,

I have 4 seedlings growing under CFL lights for now. I want to try and save up for an HPS systems since I have 4 plants going and that means a lotta CFL's down the road.

Any advice or tips from anyone who has grown with HPS systems before? How much can you really get out of a plant using them, I've heard its a lot more and I certainly want it to be worth it if I shell out the money for one.

i run 400w mh/hps but i do one large monster at a time(hydro 2x3 tray in a 4x4 tent) ive found that i get a much higher yield and over all better quality however variety suffers and there is little room for mistakes i regulary pull 10 to 12 oz per plant heavily lsted for maximum light penatration