I used to have a 450D, now I have a 7D. I will speak from experience.

The 450D is of course an upgrade to the 400D and the changes/improvements between the 400D and the 7D will probably be greater than between the 450D and the 7D, so add that up while reading this.

To begin, as you most likely are aware, lenses matter more for image quality than the actual camera does. The 18MP sensor, allows me to see the details on the photos more, and how unsharp some of my lenses are (I see differences very clearly compared to what I saw with the 12MP sensor of the 450D). So, we have more pixels, not that important, but you do get more details and most importantly, you get a greater possibility to crop images. Otherwise, regarding quality of the image, there is little difference between my 7D and my old 450D. I believe the 7D has somewhat better colors though, but otherwise, I think they are almost equal.

The 7D's build quality blows the 450D away. This beast is not the kind of plastic (but expensive) "toy" that my 450D was. It feels a lot better in the hand, and I am way more confident about it being able to take some rough handling at times.

Changing settings, such as focus mode and so on, is much faster with the 7D. It is more of a "pro" camera. It also has way more options, allowing you to fine tune the image.

The autofocus is amazing. It is faster than the 450D, and it is easier to set up, and more precise. Depending on what you do, this might make the difference between a great image and a "meh" image.

It shoots series of pictures faster too, which will matter if you photograph wildlife or sports.

I should also add that the 7D handles high ISO numbers much better. This is not that important for me as I mostly use a tripod anyway, but still nice to have when you shoot without it.

It also has nice features such as a built in gyro, so you can check whether the horizon (or something) actually is straight. You also get filming, which is nice, but I have not used that for anything serious; it takes great movies though. I know you don't care, but it is nice :).

What I really like is the built in wireless flash control. I have an external 440EXII flash, and if I wanted to control it wirelessly with the 450D, I would either have had to buy a transmitter (circa 200$) or a 580EXII (which has a built in transmitter, but ain't exactly cheap either).

As already mentioned here, the 100% viewfinder makes getting the composition "just right" easier. I shoot landscapes (and close ups and wildlife), and it really helps here.

It feels more sturdy, and it gives you more freedom. It is more of a serious hobbyist or professional camera, while the 450D mostly is a first-DSLR or family camera.