I had this dilemma also before I bought a 40D last year. The live view thing is great and the fact that no additional items are needed to use the camera directly from the laptop. It allows you to take any length exposures you like and series of images with the software provided. I can tell it to do 10 x 300sec exposures and sit back. The 40D has 14 bit vs 12 bit images in the 400D that gives greater dynamic range.
The 400D needs an adapter to use the bulb exposure via the computer.
I know nothing about the 450D but as it is not available yet and has not been evaluated it is hard to have an opinion.
As a camera to use for day to day use I have been very happy with it. I was resistant to getting a digital SLR and have been using a wonderful pentax645 medium format camera. I must admit that I have used the film camera much less since owning the DSLR mainly because of the size difference.
Go for it