Hi there Curious and welcome to the Board!
As long as the mount of your lens is compatible with the camera you are wanting to mount it to, no problemoe. If your lenses are somewhat dated, you may find that not all of the camera's functions will work but that all depends on the maker and model of camera. (You can check with the maker for functional compatibility with dated lenses.) I've got a Canon 20d and a Nikon D100 and all my Nikon 35mm lenses work just dandy on D100.
As for the magnification factor, it again all depends on the dSLR body you choose. Some are full frame (like the Canon 5d) and some have a mag factor (like the Canon 20d). This mag factor can range from <1.3 to 1.6 depending on the camera model. For instance, at 1.5x (mag factor of 1.5) a 200mm lens would equate to a 300mm lens. No change in the aperture so you end up effectually with a faster lens (200mm f4 would equate to a 300mm f4!). Nifty if you will be doing telephoto work. Yippee! On the other hand, for those who mainly do wide angle work, the mag factor can mean that you'd need to acquire a wider angle lens to get the same wide angle capabilities you had before with 35mm SLR and an 18mm lens.
I think the Canon Rebel XT has a mag factor of 1.6 so your 28-105mm lens would equate to a 45ishmm-168mm zoom and your 100-300mm lens would equate to a 160-480mm zoom lens. Cool, huh!
Hope this helps ya.
