It all depends on how big you want your plants, how much you feed them, how close you can get the lights to the plants, how big the footprint of your shade is, and lots of other factors.
Basic rule of thumb, which i know lots of people think is overkill, is
50w/ square foot. I use a 1000whps which covers about 4'x5' total of 20 square feet 1000w/20sq'=50w/sq'
Doing the math, it doesnt seem right....for a 400w anyways...only 8sq'
If you do a search on this forum, you will find a couple charts which get into lumens per distance, and other interesting stuff.
good luck