If you have the same lumens per sq ft and can keep it cool enough then it doesnt matter what wattage you use the yeild would be the same.
400w hps=50,000 lumens 125 LPW (lumen per watt)
600w hps = 90,000 lumens, 150 LPW
1000w hps = 140,000 lumens. 140 LPW
As for how many lumens you want...