I am also trying to figure out - have I maxed out the photo quality on my 40D? Have I done everything I can do - both on camera and in the process of editing - to make the best possible picture? I can't answer that with a confident Yes, because Photoshop offers endless capabilities I could never hope to fully master.
The main attraction is the 5D's low light shooting power. I use natural light as much as I can, and if there's not enough light, resolution on the 40D can get speckly.
Cameras are returnable. I could buy, try, and return if dissatisfied, though all reviews assure me that once I get my hands on a 5D, there's no going back.