Most digital SLR cameras have sensors that are smaller than the standard
36x24mm 35mm film frame.  In the case of the Canon 350D it's about 1.6x
smaller (linear dimensions).  This is sometimes known as the crop factor,
because the smaller sensor effectively crops the borders off the picture.

The effect of this is that the angle of view you get with a given focal
length lens on a DSLR will be similar to what you'd get if you used a
lens of crop factor x that focal lemgth on a 35mm film camera.  So your
Tamron 28-200 gives you field of view similar to a 49-320mm lens on the
350E.  This is great if you wanted a longer telephoto, but not so good
if you want wide angle.

Another side effect of the smaller sensor is that you get more depth of
field for a given angle of view and aperture than you'd get with 35mm
film.