Hi, the Sigma 18-200mm lens represents great value, but it's not the world's fastest or quietest focuser.
See our Sigma review: www.cameralabs.com/reviews/Sigma18200mm/
If you want fast and quiet focusing on your Canon body, you'll need a Canon lens with USM. This is the name of their quick and quiet focusing system. That said, some USM lenses are much better than others, so it depends how much you want to spend, and how far away that wildlife is!
You might be best-off getting the 400D or 30D with the 18-55mm kit lens for general use, then get an additional longer zoom for your wildlife work - like a 70-200 or 70-300 range. I'd also go for an IS model for the long zoom lens to help combat camera shake.
If you live near a camera store, ask if you can try the camera with one of these lenses and nip outisde to see how fast it can focus on people walking by. it'll give you a good idea of how well they perform, and whether they'll be up to your requirements.