hmm, is there any specific area that you are trying to improve on your 40D output?
I use 40D and 5Dc together for about 6 monthes.
Overall, 5Dc will give you better image quality. Detail vs noise ratio is noticeabely higher at pixel level for 5D, the result is a cleaner picture in all ISO. At high ISO it gives about 1 to 1-1/3 stop improvement. Another advanage I noticed is 5Dc raw files are a lot more forgiving for underexposure, you can push it in PP quite a bit. The main advantage for 40D is it is more responsive, a noticeable quicker body. I persoanlly don't really care about the menu system, live view or sensor clean.
Ask you self a few questions:
Do you shoot at long end more often or wide end more often? If you shoot mostly on long end, the stay with 40D or upgrade to 7D. The 1.6x crop factor affect FL behavior quite a bit.
Are you looking for more DOF or shallow DOF? At equivalent FL, FF body gives you about 1-1/3 stop shallow DOF, so 50f/1.8 on 40D will have same DOF as 80f/2.8. It is quite noticeable. It was the first thing I noticed when I switch to FF. If you like wide angle & shallow DOF stuff, FF is the way to go.