IF you can get your hands on Hortilux bulbs, they are AWESOME in terms of useable lumens per watt for each life stage. The MH is VERY VERY blue and I was running my 400w MH with a horti blue bulb over a freaking 4x4 footprint- that's too big, according to the common practice of 400w-3x3, 600w-4x4, 1000w-5x5- but my moms did very well.
Upgrading your bulb is a very good investment and will give you more bang for the buck than adding another lamp, even, if you were using crappy bulbs!