What camera are you using. If you are using the Nikon DSLR and you have the budget, get the SB-800. If not, then save up and get the SB-800 once and for all. It's a great flash. I had no regrets getting the flash.
Depend on body u are using. If you still using film body or higher end digital body, any DX series Nikon flash can work with it. But try to get at least SB80DX or SB28DX, which I think it will only be available in 2nd hand market.
But if you using digital body such as D70, it will only work with SB600 or SB800.
Hence do check your manual or nikon website to see which you can use.
But if you only intend to buy a brand new Nikon one, there is only SB600 and SB800 to consider. This two flashes can work with all Nikon body. But do note that some bodies don't make full usage of the flash ablilty, eg F55 (I think).
As for which to choose will depend on your usage. Do a search for SB800 or SB600, there should be quite a few discussion on which is better to purchase.