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Here is where Canon shooters have a nice advantage. There are three different ways to get into long focal length optics with a reasonable budget. The choice between the 300 f4IS L, 400 f5.6, and 100-400 f4.5-5.6L is a tough one to make, so you really need to think about how the lens will be use. I shoot with a 300 f2.8L IS and 100-400L IS and I use these mostly for wildlife photography (btw, wildlife is one word).
Here are my thoughts...
if you're going to shoot birds in flight with blue sky or distant backgrounds or you're looking to work from a tripod, go with the straight 400 f5.6L. This lens is light, sharp as glass and easy to hold. However, the lack of IS makes it somewhat inflexible for handheld photography and the f5.6 max aperture can really produce an ugly bokeh if your subject is not isolated from the background.
if you're looking for an all around wildlife lens that will do double duty for some sports, I'd suggest the 100-400f4.5-5.6L. While this lens shares the slow aperture of 5.6 @ 400mm w/ the non-zoom, the IS increases your hand-holdability and offers great flexibility. As a sport or wildlife optic, you've got the zoom to help you compose when the action gets close.
if you're a wildlife specialist (or wanting to be one), I'd suggest the 300 f4L IS. This lens is plenty sharp, has adequate IS, and really does well from a tripod. In addition, it focuses close for a long lens and can be used as a pseudo macro that will allow you to work with animals that might be wary of a close approach. Add a 1.4x converter (v1's can be had for about $150) and you have a very useable 420 f5.6 at your disposal. The only disadvantage of the 300 f4L IS is that it's not a zoom.
good luck with your purchase and regards