I have a XT, XTi, and a 40D. The XT's images are not as good as the 40Ds, but I have to admit, its difficult to tell. I think the XTis images are better than the XT, but again its not easy to tell. I can't really tell a difference between the XTis images and the 40Ds. Maybe its because they use the same sensor, or maybe its because I haven't shot with the 40D for long enough yet. It does take awhile looking at quite a few photos to start being able to tell the difference.
The biggest difference is that the 40D gets MORE good images rather than actually producing significantly better IQ. The autofocus in low light is the biggest difference I notice between my 40D and the XTi or XT. The focus is much faster and quieter in low light and you can capture more of those fleeting moments.
I've heard that the 40D's AF system is improved over the 30D's... so I don't know if a 30D would be that much improvement over the XTi, but the 40D's AF system is the BIGGEST difference especially for wedding photography. We probably get 20-30 more quality images per wedding than we did before.
Do you ever get frustrated when you see a great smile or laugh and you press the shutter but the thing won't focus quickly enough because its low light or some other reason? If you do, you'll have less of that with a 40D.
Considering the price difference of $900 dollars or so between a 40D and a 350D, I'd probably recommend getting the 350D and getting a nice lens, because the lens will probably get you a lot MORE images per wedding than the 40D would. The 17-55mm 2.8 goes for about $900 dollars. That will give you 200 more images than a kit lens
.