No, they are not the same:
As you can see, the 40D is a semi-pro level camera while the XSI is an entry level camera. The 40D has a more extended ISO range, a faster max shutter speed, 6.5 frames per second compared to the XSI's 3.5fps, and a pentaprism viewfinder.
The Rebel line are all entry level while the 10D-50D are enthusiast/semi-pro level and the 1D-5D are the professional level DSLRS.
Nikon has the D40-D90 as the entry level cameras, with the D100-700 as the mid-level, and the D3 and D3x for professional level cameras.
With Sony the a100-a350 are entry-level, the a700 is the mid-level and the a900 is the more professional level camera.
The main difference is a more professional camera offers more metering for focus and exposure control and better features.
at 3:20 AM on Mar. 12, 2009