If I were you I'd rock out to my 40D and 70-200 f/2.8 rather than put an extra $500 into the 70-300 f/4-5.6 to mount on a 5D I. It's a perfectly fine lens but it's pretty soft at 300mm and as far as "equivalency" goes a tad bit shorter and slower than the aforementioned combo. This is a great example of choosing the right tool for the job. I know you just got your 5D and probably hate putting it down (I shoot a 5D II and rarely use my 50D unless I'm shooting an event with two bodies) but in this case I think your 40D better fits the bill.