This image shows the relative print sizes one can produce given a particular dpi print resolution "requirement."
As you can see, the differences are NOT as extreme as one might imagine.
Many assume that the 16+MP 1Ds Mark II can produce FAR FAR larger prints,
when in reality...while it most certainly will enlarge nicely,
you can't produce images even twice as tall or twice wide as the
20D without dropping print output dpi.

Rather enlightening... :)