OK, I had the chance to look at some higher res files and here is what I think.
You were using a Canon 350D which is a good camera in good light. Nothing wrong with the gear you have just need to use is properly and it's important to know its limitations.
The Fokker 50 shot is good quality. You were using the right settings and the result is fairly good. I had to apply a little noise reduction, I used the built in NR in Lightroom, the same is available in PS
Adobe Camera Raw. You need to look at the photo at 100% magnification and check the noise in the shadows. The contrast is almost correct, just had to darken the black point slightly, I used levels in PS
and moved the left slider until a tiny bit of real black appeared. Exposure was good and the colours/white balance was also within limits. You need to be more precise with cropping, always use an aspect ratio between 3:2 and 4:3. The examples you posted are outside of that limit and the aircraft are too low in the frame. You need to resize your photos to 1024px wide, with the 350D I don't recommend anything higher than that just yet. Apply some sharpening using unsharp mask, set the radius to 0.2 and try an amount between 80-120. I can't tell you exactly how much you need, it's a different value for every single photo but you get used to what is required very quickly.
: here I think you made some mistakes in the camera already. With so much light there is no need to push the ISO higher than 100. In fact with the 350D it's better not to go higher than 100 like ever. Also try to set your aperture to f8, especially when the light is nice like here. Because of the higher iso the shot is fairly noisy but can still be corrected with some noise reduction software. Once again pay attention to the aspect ratio and put the aircraft in the middle of the photo. Since the horizon is visible you also have to make sure the shot is level, in the example showed above a small CCW rotation would be needed. Otherwise the photo is nice and just a small contrast adjustment was needed, same way as I explained above and the usual sharpening after resizing to 1024px wide.
I will post my edits later on today just to show you what I was able to get out of the originals. Please post your version in the feedback forum http://www.airliners.net/aviation-forums/photography_feedback/
we will be more than happy to help.
I hope you'll find this useful.
Your photos are like your children, you will always find them perfect.