I bought my wife the XTi as a birthday present a few years ago. She does use it, but I was hoping to get her to tag along with my hunting excursions more with it... no luck so far
Very similar principle with cameras as with a hunting rifle. Don't neglect the glass. I can't speak much about the XSi because I haven't researched it, but I encourage you to buy a quality lens to go along with it.
If you have a 35mm Canon or something similar most lenses (ie "EF") will interchange with your XSi. The ones more geared towards digital cameras are supposed to correct more of the color fringing and chromatic abberations that come with digital technology (I think).
The XTi is a great camera. I don't know much of the features of the more recent XSi model that may be attracting you but IMHO to buy the next gen of cameras just because it has more megapixels would be foolish. Therefore, I recommend buying an XTi (body only) and spending the money you'll save on the lens. It'd be like wanting a brand new truck for offroading--or you can get one a couple years old and spend the savings on a lift kit, big tires, etc. Not a pefect analogy because there are still plenty of the XTs and XTis floating around there that are brand spankin' new. You could also get a factory refurb by Canon. I just did a quick check on Adorama and they have some for about $470.
Also, is the SXi a full frame Canons DSLR's (ie, like the Mark II)? if not, you're going to have a sensor that is only 60% the size of the full frame sensor, meaning any EF or EF-S lens is going to have an effective 1.6x the normal lens focal length. In English, on our Rebel XTi, a 50mm lens acts as an 80mm. So a 17-55 would act as a 27 - 82.5.
Good luck.
