400w will work but espesially if you go 48 deep they will light the bottom portion but not be very intense at all and you will need to keep all sps in the upper 2 ft of the tank, zoos and some low light softies will be able to thrive down low, if you are wanting to keep sps down low also you will probably want to move up to 1000w but at the same time you may run into the problem of "burning up" anything at the top of the tank, and will definately have to run a shorter light cycle have very efficient fans or run a chiller. good luck!