Both the Rebel XSi and XTi have an ISO range of 100 to 1600 in full stop steps (100, 200, 400, 800, 1600). The EOS 40D has a range of from 100 to 3200, with the 100-1600 range covered in 1/3 stop steps (100, 120, 160, 200 etc.) and the EOS 50D has a range of from 100-12800, with the 100-3200 range covered in 1/3 stop steps. The auto ISO mode on the Digital Rebel XSi covers ISO 100-800, but is set to ISO 100 in manual mode. In the basic zone modes (everything except P, Av, Tv, M and A-Dep), ISO is always in auto mode and cannot be set manually.
The images below compare the noise performance of the Digital Rebel XSi, XTi and EOS 40D. All three cameras were set to their default settings. These are 100% crops from JPEG files, meaning one pixel on the screen is one pixel in the image. Up to ISO 400 all these cameras give very clean images with little or no evidence of noise. At ISO 800 noise starts to appear, but is still well controlled. At ISO 1600 noise is much more visible, with the EOS 40D showing the least noise, followed by the 12MP Rebel XSi and with the 10MP Rebel XTi showing slightly more noise.
Since smaller pixels usually show higher noise levels at high ISO settings, and higher pixel count means smaller pixels, its not unexpected that the Digital Rebel XSi would show very slightly higher noise levels than the EOS 40D. However, noise is still pretty well controlled, even at ISO 1600 on the XSi and it's low enough that even medium sized prints made from the images would be quite acceptable.
One feature available on the Digital Rebel XSi and the EOS 40D, but not on the Digital Rebel XTi is a high ISO noise reduction mode. This appears to significantly decrease the chroma noise while not greatly affecting the luminance noise. Chroma noise is random color variation between pixels, while luminance noise is random intensity variation between pixels. Chroma noise reduction doesn't significantly affect sharpness, while luminance noise reduction tends to soften the image. A downside of enabling high ISO noise reduction is the camera image buffer is smaller, presumably due to the increased processing power required for the noise reduction mode leading to slower compression and storage of images, so the buffer fills up faster.
Below is a sample image shot at ISO 1600. The image on the left if the full frame, and the image on the right is the area outlined in red at 100% (one pixel on the screen = one pixel in the image). As you can see, the noise level is pretty acceptable, considering that the 100% crop is equivalent to looking at a very large print (maybe 24x36") from a close distance.