If I had a Canon 450D, I wouldn't up/downgrade to a 40D, personally. If I were to upgrade to a crop-sensor camera, it would definitely be to the 50D or I'd wait and spend my money on lenses and/or lighting. If you are planning on upgrading to the 5D later on, I'd buy only EF lenses as the EF-S lenses won't work.
When you say 5D, do you mean the 5D or 5D Mark II?
I'm planning on upgrading my XTi/400D to the Canon 5D Mark II within a year. I shoot mostly landscape, but I've been getting into portrait photography and lighting. If I were to do portrait and event photography strictly, I'd go with the 50D as the lenses are lighter and easier to carry around.