[acropolite (Phil)]

The 50D, despite it's higher megapixel count, has lower noise levels than the 20D (and so the 350D). (I've done side by side comparisons with my 20D) and is as good colour and saturation wise.
Dynamic range is better than the 20D and on par with the 40D.
As for any increase in resolution, the difference from 8mp to 15.1 is bareley noticeable even if you pixel peep.

The 50D has recieved a lukewarm reception by reviewers, mainly due to the fact that it isn't a quantum leap up from the 40D, although it's on par with the 40D when it comes to image quality and the extra features such as lens micro adjustment and the larger LCD screen are a definite bonus.

The larger file sizes are an annoyance as the files are almost 3x the size for RAW than they were on the 20D making for much more workload on your PC when processing.

Here's a 400% crop comparing the 50D and 20D, as can be seen, the 50D output is noticeably sharper (but slightly smaller due to the increased pixel count) and the shadow detail is much better. CA becomes more noticeable with the 50D due to the higher resolution. The 50D crop is left, 20D to the right.