There are gazillions of side-by-side comparisons of digital cameras on the "Internets", so my contribution is not going to be profound. But it is interesting to me because I feel that our SD200 is WAY better than it should be, and my XSi is worse than it should be.
So here is the first A-B-C comparison. This morning (at 8:30 AM CDT), I took a shady picture of the deck in our back yard with each camera set to automatic, flash off, on a tripod. Here is the un-processed result from, first, the SD200:
(Click on the image to see the full res version.)
Now the XSi.
(For the XSi and the 7D, I used the exact same lens: The Canon EF 50mm f1.8.)
The automatic image from teh 7D is 9.2 MB, and this blog has a limit of 8 MB per image, so I resized it from 5184x3456 to the same size as the XSi image, 4272x2848. Here it is:
For the final stage of this little test, I ran the "I'm feeling lucky" process from Picasa on each image, and then exported them to 1600 pixels. SD200: