Better resolution - Another body or another lens?

This is a trick question, right? Since Canon doesn't make an IS version of the 400/5.6L, the choices you have provided would have to be the 5D Mk II. Otherwise, save your money by moving closer and shoot with your 70-200/40D combo.

please define "larger"...

answer depends on your definition. And just out of curiosity, would this "larger" print be for your pure enjoyment or are you actually making a profit(being monitary, awards, or just to simply satisfy the eyes of the beholder) out of the printed image?

What proportion of the frame will the bird take in the picture - in other words what size bird and how far away? How big do you want to print (assuming you are printing)?
The 50D jumps from the 10MP of the 40D up to 15MP - I am not convinced that this is a significant improvement and the high ISO performance is better on the 40D which will enable you to get faster shutter speeds. My first preference would be for the 400mm f5.6 which by all accounts is one of Canon's unexpected gems for bird photographers.

This discussion recently arose on POTN: http://photography-on-the.net/forum/showthread.php?t=700091

I agree with Mike that the 10 mp to 15 mp increase from 40D to 50D isn't that big a deal, and I far prefer my 40D's ISO performance over that of the 50D.

Mark, conversion and post processing decisions can have a drastic impact on apparent detail and resolution too, but we really need much more information about your intentions here - this isn't a "black and white" issue.

As to lenses, my preference is for the 100-400mm over the 400mm f/5.6.

The 400mm will bring the bird much closer to you. For small birds in the wild, I think 400mm is right at the minimum necessary. The 50D will give you more room to crop the image if you need the subject to fill the frame. I won't even begin to open the 40 vs 50 debate. I've noted comparisons between the 40/50 performance at different ISO settings. Believe me, you will not be disappointed by the 50D's performance.

Hi mark, First off why not take the shot (or a test shot of an object similar in size and distance as your bird) and see if the 70-200 is good enough. Try cropping the image and print. You will be surprised how good a cropped image can be if you have got everything spot on. If the print is not sharp enough, pixelated or what ever then you are going to need more lens not camera. 400mm is the minimum, either a prime 400 5.6L or a zoom 100-400LIS.

I won't even begin to open the 40 vs 50 debate

well, you just did by your biased comments :). I have actually shot with both. I decided not to purchase the 50D and keep my 40D for the obvious well regarded reasons all over the net and cnet. I did however purchase the 5DmkII(just to calm my gear acquitision syndrome GAS) and to my dissapointment the image quality is dependant of the photographer, not the camera. So, take my advice and keep your 40D and acquire a lens that best suits your budget and photographic technique(bird photograhy in this case). By my personal experience(not a bird photog, which I find to be the most boring pictures ever) on "an object shot by a distace away" I find that 200mm does not cut it. Try an ef 100-400 L lens with IS.

Coming back to the debate. I am almost sure Mr Kiessel is a very proud owner of a 50D. Well it aint that much of an advantage over the 40D, believe me... 10mp-15mp is NOT that big of a deal, even for serious cropping.

I' also have shot with the 50d&40d, keep your 40d and I would get the 100-400L IS or maybe get x1.4 good luck.

I am almost sure Mr Kiessel is a very proud owner of a 50D.

Why, yes...yes I do own a 50D. That's very perceptive, Mr. Brocanega. But how on earth did you know?