I currently use a Canon 180mm f/3.5 macro lens with a 30D body for closeup photography. Using the diffraction limit caculator on http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm would indicate that a full frame sensor would allow use of f/22 with no diffraction limit compared to f/13 for a 1.6X crop. Assuming that these results are reliable, would I obtain a noticeable increase in depth of field and resolution using a 5D body?
It depends on if you shoot from the same or different distances with the two cameras, but basically I think you'll find the smaller aperture usable on the 5D is pretty much negated by the smaller DOF of the larger format size. Overall I doubt you'll see a huge amount of difference in DOF. The 5D image should be a little sharper (if you don't need to crop it to get the magnification you need)
Thank you Bob for your prompt and helpful reply. Over the last year or so I have read most of your articles and they have given me a much better understanding of optics; as a mechanical engineer I need all the help I can get! I had not seen the 5D/20D comparison before and after reading it, it seems that the 5D would definitely be worth buying for the improved sharpness. Thanks again.