I can't recommend SB-400. It is small, but that's about it. True it is just slightly better than integrated flash if your ceiling is not too high and you shoot vertically but in most cases it limits you in what you can shoot.
Get at least SB-600. It is much better (high-speed sync, can zoom, can bounce in all directions, wireless operation, much higher power, etc.)
If on budget get 3rd party flash. Any Nission or Yonhnuo TTL flash is way better than SB-400.