I'd go the 40D route, too. I just picked up the 55-250 lens last week to photograph a parade. Worked out pretty well. Not quite as nice as the 70-200L I had on the 5D, but still held its own. This Flickr gallery at
Fremont Solstice Parade 08 - a set on Flickr has a combo of the 40D/55-250 and the 5D/70-200.