Really depends how much flash work you're going to be doing, the Nikon flashes are the best flashes around and will match up perfectly with the body. The other advantage of the SB600 is it can act as a slave in the Nikon CLS system (you'd need a master flash (SB-800, SB-910 or compatible body, D300, D700 etc to use this though).
So if this is something you think you'll use in the future or you'll be using the flash all the time / heavily then it's a decent buy. If you just want a flash for occasional use then something like the Nissin Di622 MK II ~ £110 will more than do the job.