There are a few main options you can use for this. You can use Nikon speedlights that include slave functionality with one flash or control unit that provides master functionality for their wireless optical sync.
Another option that requires purchasing a few more devices, but would allow use of your SB-400 is to buy PocketWizards. PocketWizards or similar products are third party wireless transmitters and receivers for hotshoe flashes. They aren't super cheap, but aren't terribly expensive either when compared to remote capable flashes (particularly master flashes). The advantage is that you can use them with any flash you already have and they can be radio controlled, but the disadvantage is that they are a third party system and thus are not guaranteed to be forward compatible (though the record is pretty good).