I'll try and do the comparison shots Kaz, but truthfully, I don't think the big gain is in reduced noise. The noise is definitely slightly less, but more than that -it's finer and looks different - more like film grain in it's appearance and less objectionable.
The upgrade choice is a difficult one for anyone who owns a 50D already. I'd say for anyone who needs longer glass then that should be a priority over the 7D. The 7D gives a slightly larger 1:1 image but nothing like so large as the 50% increase in pixels from the 40D to the 50D, so the longer glass would probably be money better spent.
In your case I know you have the glass already so - why should you upgrade?
The 7D is IMHO the best camera Canon have produced in years. It's not a re-hash of an existing model, it's a completely new design and it shows in every way. The way it's set up is well thought out and the handling is great. The new AF system is blistering fast. I've noticed a big increase in keeper shots - it even nails a lot of those shots where the bird moves it's head at the last moment, so the servo system is certainly faster and reacting quicker.
I spent some time in pretty grey light tracking seagulls around the sky and once locked on it maintains focus really well. I'm sure any failures would be more down to me than Canon. I've owned a 1DMK2 n and I'd take a lot of convincing that tthe 1 Series AF is any better.
Itr's lots of improvements that make the 7D what it is, some are major and some minor but the complete package adds up to a lovely birder's camera.
If you do it I'd say get the battery grip too which feels better and the camera balances better too on a gimbal, especially with the 500mm f4L
Have a look through this huge thread on POTN....
CLICKY
Lots of 7D pics by people of different abilities, but the overall quality of the images is pretty impressive I reckon.