I tried out the SB-600 and decided on keeping the 400.
SB-600 is more powerful, more on flash adjustments such as zoom etc, its head can be tilted as well as bounced. Its fairly bulky on the D40 but usable.
I still use the SB-400 and its good enough for my needs, small, light, powerful enough and does bounce, together with an omnibounce its a great bit of kit for a casual or occasional flash user like myself!!