200 mm will give you a real issue with respect to space.
considering image size = subject size x focal length / distance for a full body shot and assuming an average person of 1.7 meters in height you would need to be at a distance of 14 meters or 47 feet to fill the tall dimension of the frame. I am not sure how you calculated your distance but you should check the numbers again, I think you are out by a factor of 2.
that is a long way away!
perhaps an 85mm F1.4 would be a better option?
just my $0.02
edit note, your distance seems to be more based upon the circle diameter of a full frame (i.e. film) camera than an ASP-C sensor with a vertical height of 24mm.
as for bokeh, I personally prefer a preset lens, with a round at all apertures diaphram. these lenses seem to give better separation of foreground against background.
At least that is what I have seen with my 105mm and 135mm presets.
Last edited by Lowell Goudge; 08-28-2010 at 08:17 AM.