Like Dave I had the D70 but decided to upgrade for a number of reasons which, if I'm honest, included the fact that I wanted a new toy! The D200 is a cracking camera and even now when I occasionally use the D70 I realise that whilst it is a
very capable camera it is not a patch on the D200. The menus are easier to navigate, all the buttons are in the right place, the viewfinder is much, much better as is the screen. I also wanted 5fps for my sports shooting, particularly my equestrian work. One thing I have noticed about the D200 is that it is not as forgiving as the D70, make mistakes with this camera and they show up, whilst it was disconcerting at first it certainly improves your photographic skills.
What finally swayed it for me was the feel, I was dithering for a while until I actually held one with the MB-D200 grip fitted, it just felt so robust and well built and it suited me perfectly. I bought mine second hand and have never regretted it, I still have the D70 as a back up camera and I still use that on occasion when I need the faster flash synch speed that it offers e.g. macro work of flying insects, water droplets etc
With regard to lenses there is no doubt the 17-55mm f/2.8 is in a class of it's own and I would love to own this, the only thing that's missing from this, for me, is the fact that it doesn't have VR (same concept as the Canon IS) which I think they should include at that price. If you are looking to use the D200 for portrait work I would couple it with the 50mm, either the f/1.4 or f/1.8 as this is about as sharp as you can get and can be very useful for indoor portraits using natural light. As Soupdragon says the 105mm VR is another very capable lens and is high on my wishlist (along with the 70-200mm VR
) as I have heard nothing but good reports about it.
I know people always say that the camera is only as good as the person using it but if it gives you more confidence whilst shooting you will improve, I know I have since I got mine. The other fact you have to face up to is that the D200 is now firmly embedded in your sub-conscious and you will not sleep until you get your mitts on one.
In summary I would say the D200 is worth upgrading to, it is as near to a pro spec body as you can get without paying pro prices, couple this with a 50mm lens and you will have a superb combination which I think will suit your needs admirably.