It definitely depends on your interest. For what it sounds like you are doing now, I would recommend the d90 to save a bit and get great lenses. I currently have a d90 and have been looking to add a d300s sometime soon. The main reasons are the iso and the focal points. When I started out photographing professionally it was strictly families and my own kids with natural light. I mainly do that still, but have quite a few evening weddings and do not desire to use flash. The d90 does great in smaller situations, but with the higher iso I do see a bit more noise in the images.
The video was a huge selling point for me being a parent. I use it constantly with my kids!
Ultimately of course the choice is up to you (budget, need, etc), but in my opinion, I say get the d90 initially and enjoy that one before deciding on the extra $ for the d300s.
The D7000 is an upgrade over the D90 and is receiving pretty good reviews (http://www.kenrockwell.com/nikon/d7000.htm). However, for the price of free for the D90 vs $1499.99 for the D7000, I would go with free, invest in lenses, use the D90 for 2-3 years and then upgrade the the camera. Though if you already have the lenses you want, go for the upgrade.
It all depends on your personal preference on how you want to spend your money and types of pictures you want to take.
I own the 18-200 lens, as well as several others. I love the range on the lens, but despite covering pretty much any focal length I would want, I rarely leave it on my camera. The primary reason for this is that I find its images to be too soft (with the exception of those taken at f/8 or f/11). For some people, sharpness is not paramount, but it sounds like it is important to you. When comparing images to those taken with a sharper lens (like your 50mm f/1.8), you may be disappointed (I was). The reviews on the 16-85 are stellar, and it should be quite a bit sharper (and optically better in general - less distortion etc.). If I had it to do over again, I would probably go with the 16-85.
Klaus mentions that the difference between 16mm and 18mm is important - I couldn't agree more. It's much more pronounced than, say, between 50 and 52mm. I would also note that landscapes don't necessarily need to be shot with a wide angle, however - it depends on how much you want to compress (or expand) the depth in your image.
Another thing to note is that you may be able to get "closer" flower shots with the 18-200; I can't remember the minimum focusing distance for the 16-85, but because the 18-200 zooms in more, you can probably fill the frame more with the 18-200 (I doubt the 16-85 focuses as close as the 50mm does). If you're really into flowers, you may want to consider a macro lens at some point (don't overlook the Tamron 90mm if you do consider a macro). Depending on what you want out of the flower shots, this may be advantage 18-200. I should mention that one of my favorite photos is a pseudo-macro taken of a budding flower with the 18-200. I was able to get close enough using the zoom for the flower to fill the frame, and the softer lens worked well for this particular image.
Overall, I'd go for the 16-85 if I were making the decision over again, but be aware that the 18-200 may be better suited for the flower shots, depending upon what you want from them.
"...will I be disappointed by not having a zoom?"
With SLRs, its best not to worry too much about the zoom range of a single lens but about whether you have lenses that can actually do each job you need done. One of the advantages of having a dSLR is being able to change lenses at a moment's notice (and to also be able upgrade them when required).
The Fujifilm FinePix S2550 is claimed to cover an 35mm equivalent focal length range of 28mm to 504mm. With a dSLR, the included kit lens and something like a 70-300mm lens, you'd be covered from a 35mm equivalent focal length range of 27mm to 450mm. So, no - I really don't think you'll misss the having the range between 450mm and 504mm found on your Fuji. In fact, you'll probably be much more pleased with the better results you'll get.
"...the 18-105, or even the 18-200, will only give me about a 3X zoom, right?"
Nope, the 18-105mm has a zoom range of about 6X (105/18 = 5.8) and the 18-200mm has a zoom range of about 11X (200/18 = 11.1). A large zoom range isn't always a good thing for a SLR lens because it takes more compromises to design a single lens that can cover all that range. This is one reason photographers carry multiple lenses instead of just getting an 18-200mm super zoom lens (there are other reasons too but that would require a longer explanation).