On the assumption that your lenses can produce essentially diffraction limited resolution at the f-numbers we are considering (I haven't time to check that assumption) all you need to do is calculate your pixel area which for the XSi is, if I've got my sums right, about 27 µm². That corresponds to about the pixel size of the Nikon D2X on the interactive chart on this page
. Once you get the hang of making the chart work you can see the size of the Airy disk for various f-numbers. Ideally, with D2X highlighted, somewhere between f/5.6 and f/8 is optimum while maintaining good depth of field but you could probably stretch to f/11 without noticing any softening, especially with a bit of judicious sharpening in post-processing.