A rule of thumb that I'm sure you have heard is that, for hand holding, shutter speed should be 1 over focal length - a 300mm lens should be a minimum speed of 1/300 second.
This is complicated by 2 things in your case. the 20D's crop factor of 1.6 and the use of a teleconverter ... so:
with the 1.4x teleconverter minimum shutter speed for hand holding: 1/672 second
with the 2x: 1/960
With your 400mm lens these speeds become: 1/896 and 1/1280.
Don't think for a moment that these speeds are on the high side. Remember, you are talking about what has become a 1000mm lens! That's 20x (would you even consider hand holding your scope?)
And as Andy mentioned, stopping down a bit is a good idea. Doing so, it becomes very difficult to obtain these speeds! You will also likely be using manual focus. With all these factors it becomes just about a requirement to use a tripod - and a remote release and mirror lockup is a good idea too.
This is the level where digiscoping takes over from SLR's.
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