500D and 70-200mm Examples
To show you the effect of the 500D, below are some comparative shots taken with the 70-200mm alone, versus the 70-200mm + the 500D, at the extremes of aperture. I used flash, mirror lockup, remote shutter release, Matrix 1 (natural), Normal contrast, and auto WB.
In each case, I zoomed to 200mm and moved the camera as close as I could while keeping the wing in focus. I intentionally used a messy background so you could see the difference in bokeh.
Here are the results. Each quarter shows an entire frame, resized to be 400 pixels wide. As you can see, with the 500D, f/4 gives a pretty shallow DOF -- the wing and breast are pretty much the only parts in focus.