yes the crop factor means that you x1.6 the focal length to give the effective focal length when attached to the 20D so yes 200 x 1.6 = 320mm. the lenses doesnt change but the effective focal length does and would give the same magnification as attached a 320mm lens to a film slr
as far as im aware it has no effect on the aperture.
hence why DSLR's that crop are a pain for wide angle as a very wide lens on a film slr will be subject to the crop factor of the camera and ends up not being as wide after all.
this is great for people who use telephono zoom lenses alot (as shorter telephotos generally dont cost as much as the longer ones!!!!) but not so good if you use wide angles, which leaves you with either buying specialist crop only wide angle lenses (canons EF-s range and sigmas digital only lenses) or a full frame DSLR if wides are your thing!