Wellllll. At first, sorry for my english ;)
I personally own the 500D. That was the best choice for my beginning with the DSLRs. I bought the kit with the 18-200 EFS-IS Lens, which was also the best choice, because the you wont be happy for long with the standard kit lens. A friend of mine bought himself the 550D 2 Month ago, and we've had a phototrip yesterday, so i had the chance to try the 550D.
I must say, that photo-wise the cameras have no difference, which is noticeable.
If you want to do mooooore video things, the 550D is the one for you. But if you dont do so many videos i would definitely recommend you the 500D.
The 50D is something, you dont need for starting, believe me. At the "start" i had many problems to handle the DSLR, because I never had one in my hands before, so I think you would be even more overstrained with the 50D...so with that said and with the look at the price of the 50D I would really recommend you not to buy "Her".
Buy the 500D or the 550D, but as Lambo Llama already said, go to a shop and compare these two for your own.
(Its hard for me to express my thoughts in english in this topic, so if you didn't understood anything just ask me and I will try to explain it again ;) )