Difficult to say without seeing what/how you normally shoot. Our experiences may be vastly different than what you will experience.

But I'll give you my experience since you asked

I have the 40D currently and having used the 30D and 10D before it, I can tell you exactly how many times I've used speeds faster than 1/4000s: zero. In fact, I had to go get my 5D body out to see how fast that one goes (it goes to 1/8000 as well) because I've never hit those speeds. I've only used 1/3000s and 1/4000s on only three occasions.

I can also tell you how many times I've used ISO 3200: zero. The 40D has a somewhat linear noise-to-speed ratio up to about ISO 1250. After that, the noise gets way out of hand IMO.

I shoot mostly ambient light scenes, nature/wildlife FWIW. And I shoot a lot