Welcome to the SMN Community Jennifer.

Compared to the 40D, the 50D has on paper at least cleaner signal coming out of the DIGIC 4 as it is allegedly more powerful than the DIGIC 3 and offers better noise reduction to about a stop and a half.

Also, the LCD screen has significantly higher resolution on the 50D. I have seen side by side photos taken with of the same subject under the same conditions and the 40D images looked right up there with the 50D.