As described on this page the 5D, along with the 40D and 50D have a "Silent Shooting" mode with live view. The 450D/XSi lacks this.
I bought the 40D for this feature.
Very useful when the camera mounted to the back of a telescope.
Live with with Zoom-In on focus makes for some nice images.
In Live View the mirror comes down o Focus ( AF ) then goes back up. The shot is almost dead silent.
Of course on a telescope I do not use AF so the mirror stays up.
I doubt the 50D or 5D have anything I need. The new display would be nice, but not worth the price of a new camera.