From what I understand the 400D (5.6 micron I believe, but don't quote me on that) has smaller pixels than the 350D (6.4 micron) which is slightly smaller than the 300D (7.4 micron). In this respect the 300D has the prefered pixel size.
You will find some great info here
http://www.clarkvision.com/imagedeta...rmance.summary