yeah.. a 400watter should be plenty big enough for one plant, unless it is a friggin tree that is.. i grew two nice sized plants under a single 250 my last grow, with pretty good results.. under a 400 you should be able to support a couple of plants no problem..
i think i just saw a 400 watt club around here the other day, it is for people who grow with a 400, and it would be a good place for you to check out what others have achieved under one..
best of luck with your new toy, i am sure you will be more than happy with the results..