My daughter and I shoot weddings with a pair of D40's, mostly with the kit lenses. I also carry a 70-210 for long shots and a 50MM 1.8 for low light. I have rarely felt limited by the cameras at all, and our clients have been happy with our work. Most of them end up buying albums with 10x10" pages, so some of the photos will be printed at 10x20". 14x20" prints look fine. I carry my wonderful old F4 as a backup, and if I know ahead of time that a large print like 20x30 or 24x36 will be needed I use the F4 loaded with Portra film. I also use the F4 for the group jump shots because of the much faster frame rate. You can see some of our work here: www.rogerenglephotography.com
The D40 does its job if I do mine. I still have much to learn after shooting 43 weddings, and I think it will be a while before I outgrow the D40. The only reason I might upgrade is that quite often some of the wedding guests have bigger, more expensive cameras than I'm using.