You don’t need a nikon flash, but you do need one which is compatible with the Nikon i-ttl system to get automatic flash metering.
The sb600 is less powerful than the 900, and it can’t act as a contoller in a multi flash setup. It also doesn’t zoom as much to cover as many focal lengths. On the plus side, it’s a good deal smaller. I have one myself and it’s been more than adequate for most uses, although more power wouldn’t go amiss occasionally. You can compensate a bit by kicking up the iso a bit in a lot of circumstances, though.
Mr. zincprincess told me that he wants an external flash for “our” D5000 as a father’s day gift. Love him. It’s not a bowling bowl with “Homer” on it but it is close. Having bought it, I’m not sure that I can wait until Sunday to play with it. I guess I’m supposed to let him have a go with it first, right?