Originally Posted by Cmike2780

A off camera or at least shoe mounted flash is essenstial for indoor, even daytime outdoor work. It lets you control how objects or people are lit in the scene, which can mean the difference between looking like a snapshot or a work of art. If you want to up your photo work quality, a speedlight really improves it because you are less reliant on available light in the scene.

I use a SB-900 & a SB-600 as a slave strobe. I love how easy Nikons are when in comes to wireless speedlights. I would agree with Parkerman, the SB-600, even used is a much better flash, more powerfull, than the SB-400. Its well worth the extra expense. The newer SB-700 is probably out of your budget. The SB-600's also have "gels" you can buy that can match the lighting conditions and doesn't mess with the white balance. If the SB-400 is all you can afford to spend right now, its still worth it because the built-in flash is just about useless.

Also, a lot of people overlook white balance and just set it to "auto". Everyone should learn how to do a custom white balance with a grey card (very inexpensive, accurate and saves you a lot of time in post editing).