A few clarifications from someone who has used a XTi tethered to a notebook PC: while the XTi has a composite video output, it shows pictures only after they have been taken. To see an image from a Canon DSLR before the shutter button is pressed, you'd need a newer "Live View" model, starting with the Digital Rebel XSi. The DSLR Remote Pro web site lists the 1D Mark IV, 1D Mark III, EOS-1Ds Mark III, 5D Mark II, 7D, 40D, 50D, Rebel T2i, Rebel T1i and Rebel XS as other Canon models with remote Live View through a data cable tether.
Also, from direct experience: it takes little time to get images from a tethered XTi to a USB 2.0-equipped computer. Using either DSLR Remote Pro or Canon's combination of the EOS Utility and ZoomBrowser EX, it takes no more than two seconds for the camera to send a JPEG image to a computer, and around six seconds to send paired RAW+JPEG files of around 15 MB combined.
DSLR Remote Pro and tethered photography works very well on the XTi, but it's unreliable and tedious with the older Digital Rebel 300D. Pictures need nearly a minute to transfer, and often, images would be lost when the 300D was tethered to a computer. A newer model camera will work better.