Neil,
what type of "action shots" do you anticipate shooting? Moving shots of dogs in the show ring? Shots of dogs at Agility or other Performance events? Dogs running around in the dog park?

I ask because for some activities, you should be using the equivalent of the Canon 1D MKIII but for others, any good camera -- and the D300 and E3 are excellent -- would work just fine.

For most of the static shooting, the Sigma should not only work well but give you very nice color.

Jim