Pros for the 50D over 20D:
Faster drive- the entires action of the exposure is faster, Mirror drives up almost instantly, and goes down just as fast. My 20D was not slow at all, but that extra 1.3FPS is definably faster.
Larger burst- I have yet to fill up the buffer with my extreme iv sandisk cards in it, even at 3200ISO and RAW. I was getting around 20-24 shots per burst and it I never saw the remaining counter go below 9 the entire time.
Screen to die for- You can see so much in the field and you have lots more options for using review(show AF point, histograms are much easier to read, RGB histograms).
Uses my mountain of batteries and my old grip from my 20D.(this is one thing over the 60D)
Angled top screen- this is not huge, but definably nice to use without moving the camera all the way down to see it.
AF-ON button- I use it every now and then for sports shooting, but if you turn off the shutter release to do AF you still retain AE/FE lock.
Though a good telephoto lens with a USM motor will make your keeper rate go up more then a new body would. But if you have a good lens, the AF is much better then on the 20D.