This box might not be tall enough for a 400 watt. I have about 5' of clearence for the light, plants and pots in my small setup and I wouldn't mind 6-8" more. I reccomend checking out the thread: "Lovely Ladies" by justaseed he has 3 SCROG'd white widows and gives detailed instructions about how to train and grow them, it helped me alot. As for yield 200g is very reasonable to expect. If you haven't built the cabinet yet might I suggest a C13 or C25 from Wal Mart, they arn't very expensive and give great results.