I changed from the 450d to the 50d. What a great camera it is. Totally different feel, much chunkier and more solid feeling. The LCD screen is a dream, great resolution and realy clear.
I spent some time with my mums 400d over Christmas, it felt like a toy compared to the 50d. The ISO on the 50d is far more useable, definately what you need in this dreary UK weather.