Hi Oldskull*!
*joke :tears:
mmmh, the SB-900 is here in Gpoint 200€ / 240$ more expensive then the SB-800, what is a phantastic flash.......
You have to decide yourself if you want to pay for this:
+ Update possibility
+ DX-/FX-Mode (D700!!!!)
+ Lightingtempolatese (Standard/Flat/Middle)
+ enhanced Zoomreflecor range
+ sealed
+ Overheat- and wetness protection
+ automatic recognition of used filter and whitepoint adjustment
+ a real on/off switch (as it looks on the pics)
+ loads with 4 AA-cells as fast as SB-800 with 5
that price! But in THIS case you also should think about another camera

D700 for example?
it would be double price to buy a SB900 here in Gpoint instead of a like new used SB-800
All the People talking about the oldschool flashes: they only work manual and all iTTL advantage is for the cat (as we say her in G.)
But:
I gotta warn you... the D200 has an annoying way to flash, it is set to do foreflashes, and didn't find out how to shut this off... not even calling Nikon helped.
Anyway, I'd get me a used SB-600 or SB-800, with the 5th cell adapter and bouncer (If I hadn't already...)
But is the "lots expensiver" warranted on the 900?
Sometimes when a new model comes out, it has features and abilities the one before simply doesnt have.
I realize most people dwell on price, but if Im going to spend cold cash on a flash Id really like to get her what is the best as she will be keeping/using it for quite some time. I guees to me, once Im in the 350+ price range, I dont mind spending another 100 or more to get the best if that is the case.