[Utah-astronomy] Purchase advice on Canon Digital Rebel XS, XSi

I am getting ready to buy my first DSLR and I would like to buy a Rebel XS or XSi. This would be a general purpose camera but I want to cross-purpose it as an astronomy camera. I feel the cheaper XS model meets my needs. I am unclear on: 1) For time lapse night time photography of a large sky section with the 15mm lens, how do you get the images off the camera in real time since it only holds 4 shots? 2) Other advice of comments are the purchase are welcome. Thanks - Kurt