Which Canon digital camera body should you buy in order to use old lenses from a Digital Rebel XTI?

Any Canon camera with the EF-S lens system will work. That includes but isn't limited to all of the rebels (XT, XtI, XS, XSI, and the new T1I).

The only cameras that WON'T work with your Rebel XTI lenses are Canon 1Ds, Canon 1Ds Mark II, Canon 1Ds Mark III, Canon 5d, Canon 5D Mark II. Another other Canon digital camera will do the job. The reason that these won't work is because they are full-frame cameras, which need a larger surface area of the lens and will not accept the mount from the smaller EF-S lenses. These cameras are all professional cameras and cost a couple thousand each.