Another what should I buy lens question.

--I currently have a Canon  75-300, IS USM as my longest lens.  I would like more reach, at least 400mm.   What is the best bang for the buck in this range.  I know, I know.,  And if I had the bucks there would be a Canon 600 L  in my bag.  Unfortunately that isn't in the budget, in the forseeable future.  But I might could squeeze in something a little less expensive.   What are the suggestios/experiences of this group?
I want to go to my grave, tires bald, fenders knocked off, out of gas, and yelling, WOW what a ride.