The only lens I have is the 18-55 kit lens and i don't have a real budget to buy some expensive lens this is why i look for a kind of ''all purpose lens''.
Pentagon 135mm M42 lens or similar, with adaptor is a cheap alternate and plentiful on ebay.
I would buy a 55-200 zoom before I purchased a 50mm prime whose focal length you all-ready have covered
I said before, the shooting distance is important. at the long end of the 200mm lens, at the right distance, you can isolate a subject beautifully (and you have the advantage of a longer zoom in your bag too)
halli is there a quality difference between the fd with mount ring and the ef?

Nice shallow depth of field can be achieved with your 18-55, I got this shot from mine (it's not the most interesting photo but it was for a comparison) - I do think it shows dof & bokeh well enough though (no editing). The next lens I bought was the 50mm macro as I liked the option if getting up close.
test shot 2 by Iris_255, on Flickr
I think Jen, that had the subject (ladybird) and the nearby leaf and stem been in focus, and the rest been totally out of focus, then that could be classed as "bokeh", but the background in that shot is still recognisable...
The lens used, was the Minolta 70 - 210 f4