CANON EOS 400d/300d/500d  

I have a canon 1000D and I love it, it is so easy to use, I would certainly recommend anything from Canon, I think they are a very reiable company and their cameras are great.

I have a 450D, and I'm extremelly pleased with its quality!
The difference between these you have mentioned isn't so big, but I recommend to you the 500d. It's the most expensive, but it has the biggest resolution (15,1mpx) and it can record movies too, in 1080p!

The 40d is an amzing camera. I'm a professional level event photographer, but don't work large enough to drop 10s of thousands of dollars on equipment. The 40d works well in low light, you can run the ISO up without any problem. The 6.5 fps shutter is another great feature, as it really helps to catch that one action shot you're looking for. I completely recommend this camera.