The top of the line Nikon SB-900 would be a great choice although a little pricy. The new SB-700 looks interesting but I haven't checked it out yet. Much more attractive pricing though. For around $220 US the SB-600 is an excellent flash although the rather arcane menu system takes some getting used to. Depending on how much $$ you want to spend I'd get the as much power (higher guide number) as I could afford.
Flash power is great but the key is to learn to use the flash balanced with ambient light. I like to use my SB-800/better beamer with the D300 set for high speed sync and fill at -2 stops. High speed sync allows limited flash use with much higher sync speeds than the 1/250th speed. This works nice for me in most situations but if need be, I will change the fill setting one way or the other. I keep the high speed sync set all the time. I rarely use my flash at full power though, so I feel that the capability to do the above is more important than raw power! The SB-900 would be really good but as I mentioned the SB-800 is fine. I don't know if the other flashes mentioned in this thread will allow high speed sync and/or auto fill. Good luck and practice!