Mark U , Mar 23, 2006; 09:01 p.m.
Divide your favourite 35mm focal lengths by 1.6 to get the same angle of view on a 20D. If you want the same narrow depth of field capabilites shooting wide open, you should be looking for lenses that are 1 1/3 stops faster than your 35mm ones. You will find that diffraction limits sharpness at wider apertures too - if f/16 is your diffraction tolerance on full frame, you'll get a similar effect at f/10 on the 20D.
Bob Atkins has a nice discussion of the consequences for depth of field here: