Like a lot of photographers here, I'm looking into possibly upgrading my camera in the future. The digital SLR market seems to have exploded. And newer and newer cameras are coming into the market. So to decide what camera to buy next is always a difficult choice. Cameras with larger megapixels, faster frames per second, larger LCD, sensor clean, etc. etc. This is my thought process that may help people decide. First of all, ignore all the "new features and specs". Yes, more megapixels is nice. But do you really need more. Larger LCD is nice also but as a tool for you photography, it doesn't really give you that much.
I think what would be more useful is to select it base what "photographic" advantage it gives you which will produce photos you can't with your current camera. Also, whether or not the camera actually gives you more "tools" to work with.
I currently have a Rebel XT. I feel if I get a second camera like the XTI, XSI, or even a 40D. It really doesn't give me much more "photographic" advantage compared to my XT. However, if I decide on a 5D, there is much more difference. Why?
1) 5 D is a full frame, so much better noise performance.
2) You get 1 stop more bokeh (background blur) with all you lenses. i.e. your F4 lenses give you and equivalent of F2.8 lenses on your crop frame camera and your F2.8 lenses are equivalent to your primes on your crop camera. Which is pretty cool. So not only your camera is upgraded but all your lenses are also.
3) one lens does double duty or two focal ranges. I.e. 17-40 is a standard zoom on your crop camera but and ultrawide on a full frame.
So consider these points when thinking about upgrading. I hope this helps with your upgrading.