So, I just got a camera, a Sony A200W, and I'm wondering if I should have opted for a Canon S5, a Nikon D40, an Olympus SP-570UZ, or a Canon A650IS. Does anyone know?
That's kind of like asking the better choice between a bicycle and a motorcycle. Same idea, very different implementation and usage.
The Nikon D40 and Sony A200 are SLR cameras, which means a bigger sensor, interchangeable lenses, a much greater degree of control and overall faster and better quality in every aspect.
The other three are point-and-shoot cameras that are designed to let you take pictures without making you think.
Between the Sony and the Nikon, you're not going to see much of a difference in performance. Keep what you have, it's a fine choice.