I and several others here shoot with the Nikon D80. It is being replaced by the D90 so prices are coming down.
I'm not sure how it compares to the xsi but am very happy with it.
Research as much as you can about which system you want to buy into by thinking about the kind of shooting you'll want to do now and where you see yourself going and then check into the lenses and other things offered by each brand.
Don't forget sony, fuji, olympus, pentax which all also have good cameras.