I can't speak for the 7xx series, but for my camera ISO 400 will capture 3x the amount of light as the 100 setting. Offsetting that advantage is the increased amount of noise that's present with the faster ISO setting, and I have had only limited success cleaning up the noise post processing. The best deal is to use a good flash, IMHO, and try to shoot at 100 or 200 ISO for best results. However, for action shots in low light, you may not be able to stop the action unless you allow the camera to select 400.
As for depth of field, you can have very good success blurring the background using the widest aperture. For really spectacular, short DOF shots, use the macro mode on your camera.