What camera would work for me?
I currently use a Nikon D50 with a Nikkor 18-200mm VR. I love this lens… I mainly shoot pictures of my dog, dogs I foster, and other dogs that come through the rescue I work with… I have also recently gotten into outdoor portraits, and wildlife(wildlife is the main reason I got a lens that would go to 200mm, and some day I want a lens that'll zoom even further). Some day I'm going to get 1 or more off-camera flashes and start working with backdrops and all that. Some day!
I almost never use the on-camera flash.
My D50 just can't handle noise well. Any picture taken at an ISO above 400 has to be trashed because it is way too noisy.
I'm ready to upgrade my camera, and need one that performs very well at higher ISO's. I'm going to stick with Nikon. What would be the best Nikon camera body for me?
Added (1). Drew, I'm not overly concerned about price. I'll buy used.
I've heard bad things about the D3200's ISO performance…
How about this. Set aside a hard figure for your budget. Make a list of the Nikon models that match your budget. Then buy the camera that falls in the middle of pricing list. Your camera is a 2005 model, anything after that is an improvement. The D3100 and D5100 are still popular. I can list a bunch of cameras but that's pointless when you can't afford it. Just buy one you can afford but don't clean out your budget for it.
If there's a National Camera Exchange near you, buy there. They will let you return it without restock fee (in mint condition of course) if you're not satisfied with its performance.
The new D7100 is awesome.
The D3200 will do all of the stuff that you want, is easy to use, and isn't terribly expensive. That's the one I'd recommend. The high-ISO performance is much, much better than the D50.
Edit:
If you're willing to spend more and buy used, the D7000 is a great choice. I've shot with one, and it really looks pretty clean at ISO 1600, as good as most full-frame cameras. The reason that I wouldn't recommend a full-frame camera like the D600 is that you get 1.5x the effective lens length with the smaller sensor, which is better for wildlife.